could i use a store bought graham cracker crust

Maybe- I'd think it would have a strong tendency toward sogginess. Up to you if you think the flavor would be appropriate. If you decide to try it, watch the sizes- I think these things tend to run to small and shallow, though I don't use them myself.

I just consulted the Meta Given cookbook to look for a mincemeat question and she recommends you buy a jarred product since there are so many good ones on the market - and it is much easier than making your own. I say that, because I don't think that she personally would be against you buying the pie crust you would enjoy using. Graham cracker is different than pie pastry but still good.