Basically it's like that, the only thing really needed is a big enough rock to hit Mars and by chance the correct trajectory to make the Mars rock hit the Earth eventually. By the same token, isn't it interesting to think of where all of Earth's rocks might have ended up?
Here's a little tidbit for those interested: one of the Carnegie museums here in Pittsburgh has a Rocks & Minerals section, and in there they have a piece of one of the Martian meteorites. You are, for the record, allowed to touch the rock. It's like the moon rock in Washington DC in that sense, but I prefer our Mars rock!