This may be possible by checking and adjusting the configstrings. The fireteam configstrings (CS_FIRETEAMS) are typically at the end and contain the fireteam members formatted in a bitwise pattern. You can view configstring information on your server with the commands csinfo <id> (id is optional, it lists all strings). One such configstring may look like:
Code:
\id\0\l\0\p\0\c\0000000000000005
in which player slot 0 and slot 2 have occupied a place in the fireteam. Note that the id equals 0, which means that this is fireteam alpha and the leader is set to player slot 0. Check the leader's team by using entity information: sess.sessionTeam is your friend in this case. By checking his team you automatically know the fireteam's team as well.
You may also check my implementation for WolfAdmin for reference: click here. It contains some functions to parse the configstring, but it doesn't allow editing. Lastly, I have not experienced with editing these configstrings so I do not know the effect of doing so; it might not work at all or have strange effects.