I would think no, but I guess it would be something the players in the specific game should decide. That would be a way for empires to "bow out" without changing the balance of power tremendously for the remaining empires. The TF would of course be free to sell some of these planets to the attacking race who caused the race to surreder, or simply put them up on the open market. Doesn't really fit the TF profile though to keep them. Well maybe a few.
My impression of this is that the TF either has no real game winning objectives, or at least doesn't have the same objective as the other races. They are there to assist in making the game more interesting for the players, and hopefully interesting for the TF player. Like a "Dungeon Master" sort of, but with less control of course. Otherwise it will be tough to find people willing to do it. A lot will depend on the person and how well they can get "in character" of the TF and avoid taking a personal interest in the conflicts going on around them.
Geoschmo