Downtube shifters to ergo shifters? Possible?

Nothing. I'd probably still be using them if my bike hadn't been stolen which forced me to buy a replacement which had Ergo shifters.
TBH Ergo/STi systems are so specific to each model/year that unless you are prepared to change the whole drivetrain, it's probably not worth it.

Sorry, but that's incorrect, at least for Campag. All 9/10 speed systems are compatible.