It's not possible at the moment.
Scientists and engineers are not units, but they are also not items (resources), so they can't be recovered from a mission.
But I can add a few flags here and there and transform flagged civilian survivors into scientists, engineers or even parrots if you wish...
Would anyone be (seriously) interested?
I certainly would! It kind of strains the "realism" of the setting for me that you can just walk into the local settlement and hire as many Brainers as you want for just cash (admittedly a lot of cash - lore-wise my rationale is that most of the initial hiring cost is finding such a smart person in the first place). If Brainers are such a valuable resource for settlements and factions, who out there is willing to sell unlimited supplies of them to our Gals?
What I'd really love is an ability to turn some captives (hire them? brainwash them?) into Brainers, similarly as to how you can turn captives into slaves right now. It would make sense for "smarter" captives to give you more brainpower for your base, so a captured Academy Engineer might turn into 2 Brainers while a Church Maiden might yield 1 Brainer. You might be able to turn this into an income stream like slaves - convert your captives to Brainers and make a little additional profit compared to just ransoming them.
Now I'm not altogether sure about how this would mesh with current game balance, as research speed past the early game is limited more by research buildings in your base instead of acquiring Brainers themselves. This might make the early game drag on a little more, and stress even more the desire to capture people alive instead of killing them.
EDIT: Maybe mid-to-lategame you might want to get a LOT of Brainers without having to capture them, and I think there could be a technology that enables that - maybe you research a contract with the Academy to buy Brainers from them? Or you could have some kind of brain-enhancing chip that when implanted into a Runt turns them into a Brainer?