It's in a stable orbit already. The solar system is pretty crowded, so it would be quite hard to add an extra planet without disturbing the other planets. It's more or less as full of planets as it could be and keep them in nice stable circular orbits. So it's just as well it stays out there. Indeed there may well have been other planets that got ejected from the solar system, hit the other planets etc. May even have ejected gas giants. What we see may be the ones that are left at the end of that process.
The whole thing quietened down after a few hundred million years. Finally ending after the "late heavy bombardment" about 3.8 billion years ago with some big objects still left for a while after that. But by 3 billion years ago the solar system had quietened down and was much as it is now.
In other words, its opportunity to come into the inner solar system was well over three billion years ago. If you wanted to do it now, well I think you could try making it into a double planet or a moon of one of the other planets, that would be the best bet. E.g. try to make it a new huge moon of Jupiter, and sneak it in quickly past the orbits of Saturn, Uranus and Neptune. With mega technology perhaps you could do that?? I think that would be more likely to succeed than trying to make it into a new planet in its own right. It wouldn't add that much to the mass of Jupiter, so the combination of Jupiter with an ice giant moon like this might still be a stable solar system (you'd need to do a model to check this).
But that's assuming some vast amount of energy available. There are ideas for shifting Earth's orbit by using an asteroid in flybys of Earth and Jupiter. I don't know if a similar approach could move this planet using e.g. dwarf planets instead of asteroids to move it? The idea was just to move Earth out a little, over billions of years. This would be a far bigger task than that, larger object, much larger change in its orbit.
Incidentally their hypothesis about where it came from is that it came from closer to the solar system originally, got ejected from the solar system, but because this happened early on when the solar system was still forming, it got slowed down by the gas and dust and ended up in this orbit instead of leaving the solar system already. If that's true (and if the planet exists at all of course) then it actually started off closer to the sun and was ejected and ended up where it is now.
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