It's the same process as becoming a regular Space Marine, but with a few additional organs to reinforce their body and increase their size. So it comes with the same restrictions and drawbacks of regular Space Marines. It's harder to become one as you get older, and it was already quite dangerous, so most Inquisitors won't bother trying. Luther and Kor Phaeron are the best examples of "old" Space Marines and they're more like half-Marines because they were too old to become full Space Marines. So with luck, you could probably become a half-Primaris. Since Inquisitors already have power armor and even Terminator armor, there's little incentive to risk your life like that.