Can I add rice to my slow cooker? 20 minutes at simmer on stove + slow cooker on high?

Pot roast. Ran out of potatoes and thought perhaps I could toss some rice in.

In that case, I suggest serving the pot roast on top of warm cooked rice. Adding rice to the pot roast in the slow cooker would make the rice mushy and unpleasant. No need to ruin a good pot roast ;)