[PvK]

Yes, both sides can keep adding troops from outside the planet. If the happiness modifiers somehow prevent the planet from rioting, then the defender can keep building them too, of course. This could turn out to be a useful advantage for Emotionless races.

I haven't tested what happens if a human player (or whatever) orders ships to lay waste to a planet with friendly troops on it. I'll have to check that out.

PvK