Probably not. Unless they were related to Earth life, then probably not made of Earth type DNA. Even DNA could use different bases. Also the proteins can be different for the same DNA if the way proteins are encoded is different.

And even also here on Earth then many lifeforms are poisonous as food for other lifeforms. Sometimes through evolution to avoid being eaten. Sometimes, it seems just accidentally. Inorganic chemicals on other planets could also be poisonous to us though tasty to ETs that live there.

The Star Trek and general science fiction movie convention where you can usually eat the food, breath the air of another planet is probably very unlikely.

Of course we can't say for sure until we actually discover ETs. We only know of one type of biology that can evolve in our universe. Whether even this biology can evolve anywhere else is unknown. So it is far too soon to know for sure if there are other alternative biologies either. Some have suggested that this is the only form of biology that can ever evolve in our universe, that in various ways it is optimal.

But the problem with that argument is that life is so complex, there is no way it could explore the entire solution space of all possible forms of life in order to reach an optimal lifeform, metabolism, method of reproduction etc. Can only be a "local optimum", an approximate maximum found by a partial exploration of the mathematical solution space of possible life biochemistries, metabolisms, methods of reproduction etc.

So there is also a chance of many other forms of equally "optimal" lifeforms, which to them would seem "best possible" just as our biochemisty seems to us - but might not be identical to ours.

And if it's true that life can evolve in many different ways, seems no reason why life evolved independently should be safe to eat. And even if there is a common origin or it evolves in identical fashion, the life could be as poisonous to us as chocolate is to dogs and parrots, or  nutmeg and avocado is to parrots, or "deadly nightshade" or wisteria seeds is to humans. Or the way that BMAA produced by green algae may cause Alzheimers in humans because of its similarity to an amino acid L-Srine  - though surely not adapted to do this, just an accident.

And - if the life is different from Earth DNA it's also possible that our defense systems which have evolved over billions of years to recognize and protect us from DNA based life might simply not recognize it as a hazard. So we could also be in danger from non DNA based microbes. Or the microbes could eat, or destroy other lifeforms we depend on, or indeed out compete them. E.g. suppose it is a lifeform that is better at photosynthesis and metabolism than our green algae, and occupies a similar niche, only slightly better, maybe totally different method of photosynthesis, as different as the method used by the purple bacteria or the sulfur bacteria are from the chlorophyl based photosynthesis - it just needs to be very slightly better than our equivalent lifeforms to out-compete them. And then if it produces chemicals that DNA based life can't use, doesn't recognize, or finds poisonous, that could be an issue too. Microbes like that introduced to human habitats or ecosystems could be devastating, and might be no way to stop it.

We could eat simple chemicals like some sugars and salts, for instance, alcohol also - but would need to take care with those also until tested as many may also be poisonous, e.g. perchlorates in the salt,

For many more details, see also my answer to:

What are the odds that alien food from an Earth-type planet would kill us?

which I made into a Science20 blog post as What Food Can You Share With An ET?

This is also why I think we need to take great care about returning any sample from any other celestial body to Earth. Including Mars, Europa or Enceladus. We could be talking here abotu more efficient lifeforms, perhaps an order of magnitude smaller than the smallest known living cells based on DNA, and it's really hard to protect against unknown dangers. In biohazard laboratories, which is what we are most familiar with, you have a known hazard, to be contained, inside the laboratory. Can we contain an unknown hazard, one not even based on familiar biochemistry, when we haven't yet studied it?

I think we should study "in situ" first, for safety reasons, as well as for practical reasons.

And if returned to the Earth Moon system, then unless sure in advance what we are returning should either sterilize it - or else - return it to telerobotically controlled facilities, perhaps a few thousand kikometers above geostationary orbit - that humans go nowhere near, until we know for sure what we have there.

Because there are too many possibilities of containment breach once you return it to the Earth atmosphere. Including things like terrorism, accidents, carelessness, over eager experimenters ignoring protocols etc.

The first samples returned are likely to be only of geological interest anyway unless we have already found life on Mars by then, because Mars gets lots of organics from micrometeorites - the organics already found are believed to have come from micrometeorites. Though there may be both past and present life on Mars, then it's likely to be hard to distinguish it unless already able to detect life in situ on Mars.

Of course entirely possible that the life is harmless, that we can eat kilograms of the stuff no problem, even if totally unrelated to Earth life in origin - but you'd want to be very very sure about that, before doing something such as introducing it to an Earth habitat or trying to eat it. Even if the risk of it being harmful is tiny, it's a risk we can't take with billions of people.

See my Will NASA's Sample Return Answer Mars Life Questions? Need For Comparison With In Situ Search