I'm looking to make a ripple cake, like a marble cake, but not with chocolate, probably raspberry or strawberry or something like that. I will be using a boxed cake mix. Do I just take some of the batter and mix it with say, raspberry jam or something and then cut it in? Thanks!

Yep, that's what I have done. Works well.
You can also just use the jam (or sleeved filling) w/o mixing it into some of the batter for a more pronouced flavor.

I used fresh fruit in a marble cake. used 1/3 of the white and folded in fresh raspberries, be gently just bust the berries up a little. It turned out fabulously. Good luck.