Pipi
Does Ryzom currently support NPC's joining your Team?
If not, is this support possibly coming in a future release?
As Ulykis mention this isn't currently possible for players and I don't know if it is on the short-term radar. There are tools available for running events (GUS) and there are tools being developed to assist in running events but I'm certain this isn't your end-goal. You're talking more about "hiring" NPCs to dungeon crawl with you, correct?
Pipi
If not, is it possible to modify the Game to support this if I am using the
open source code to make my own game?
It is possible. All of Ryzom (with the exception of the world and account data) is released to the open source community. Art assets are CC-BY-SA and all of the client, tools, and services are available through AGPLv3. There are a few projects working using Ryzom Core and there's a community dedicated to Ryzom Core. I encourage you to come join #ryzom on irc.freenode.net and chat with us. If you don't have a specific game project in mind you could always work on adding the feature you want to Ryzom Core. The work we do on Ryzom core is merged with the Ryzom game and work they do on the game is merged back. This means as long as your feature doesn't imbalance the game in any way it has a chance of being used in the full game.
If nothing else - if you're truly focused on your own project - we're a great resource of information. A number of us have been working on Ryzom Core since it was released to open source and a few of us pre-date this to early 2000s when just the core NeL internals were released.
I hope we can work together, I hope we can help you out!
Thanks!