I'm not sure if it works this way on the time capsule, but it worked on my external.
When you add a movie (or any media) to itunes, while dragging it in, hold the option key.
This will add it to itunes but the movie itself will remain on the external. (Basically it will be organized, look and function as normal, but the data won't be added to your main HD)
So for example (I'm just gonna pick two random movies):
Say you have Zoolander on your mac and Superbad on the time capsule.
Zoolander is physically on your mac and so there are no problems. itunes looks for it on your comp
For the other movie:
Open your time capsule and while holding option drag Superbad into itunes.
It should add the movie info and seem as though it's on your computer, but it's just linked to where it is on the drive.
If everything worked (and as long as you're connected to the network) then you should just be able to stream both movies to your apple tv thru itunes.
Edit: Sorry, this is all assuming that you're using your time capsule as a networked drive and not just for backups.
I hope it makes sense..