How to invite over an alien that used to be a pollination technician?

I do have one idea. Find the sim you want to invite over, but can't, also find another sim and move the two in together. Then merge the one in with your sim, while leaving the other one behind. Then do what ever it is you want for them to do. Then afterwards, go back into Manage Worlds, and move the alien back in with that other sim you had first merged. Now why said all that, is so you don't get an addition 20,000 semolians added to your sim's family household funds, but you can still have your sim enjoy the other sim's company.

I don't have an answer to your question, so I'm speculating about work-arounds.

If you're willing to play more than one household, have you tried playing Ms. P'Tech and inviting your sim over to her house?

If you don't want to play more than one HH, you can use a cheat to force P'Tech to appear. These are copied from TwistedMexi's cheats on reddit (ETA: the cheat code's now italicized for clarity):

1) Get sim ID: sims.get_sim_id_by_name Note you're only interested in the long numerical string at the end.

2) Force Sim onto Current Lot: sims.summon_sim_to_zone

Good luck!

Thanks for the answers guys, I actually figured out I could use the MCCC Summon Sim cheat on her and it surprisingly worked. But both of your ideas were good ideas too! :)