I think you need to work up to that type of training.
but
Which Bill Starr 5x5 are you doing? Advanced or intermediate.
I know what you are thinking about because I've also been down this road of wanting to keep the big 3 numbers up and still do MMA work. Problem is, something is going to give at one point with the progression and pace of both 5x5 programs, the advance being the quickest to overtrain. Have you heard of Matt Wiggy?
check out his site, it could give you some ideas.
http://www.workingclassfitness.com/
also check out my journals since I've been trying to come up with a similar routine. I've had some luck but right now I haven't been doing much heavy lifting since my MMA training has been bumped up so much. I'll be incorperating weights again very soon, well as soon as I can heal up.