Of course! If I signed a contract with /ForeverAlonePrick, then I'm obligated to keep that channel with said network. However, I have another channel, /stuffiscarrots, that is not with the network. I'm not obligated to give it to them. I could also conceivably partner it with another network. What's stopping me? People experiment with different networks all the time.
Why does fullscreen want to partner you now? If it's for the high view count, just encourage your viewers to switch over to the new channel. Fullscreen has done this before, where it accepts someone on a new channel, just taking a chance. If you don't do this, you have no other options, other than to beg Machinima to release you.