Ok, so it is possible through AppleScript, thank you!
I need to move data back and forth between Merlin and Asana and other systems and I'm surprised that even if I drop a CSV into the project that has custom field "Last Modified" already and the CSV also has custom field "Last Modified" - I still can't put data from one to the other, because on import you can't see mapping into that field, even though it is already there and it would be so much easier to just import into it. I don't understand why I can't do that (every custom field has type too, so it's another extra step) and need to go all the way around, but at least it's possible.
All best,
Vlad