Replying to Time signatures on Garage Band

Hey,
I'm recording some quick demos before i go into a real studio. I usually use protools, but it's not available to me right now so i'm forced to resort to garage band. Many of my songs use multiple time signatures such as 4/4 to 6/8 or 4/4 to 3/4, and I can't figure out how to set the metronome grid on the time line to change time signatures midway through a track. Is this even possible to do with garageband or am i SOL?