Yes you can keep the spaghetti sauce warm in the crock pot.
When keeping the spaghetti sauce or any other sauce warm in the crock pot just turn the heat setting down to the low or keep warm setting.
You don't want the crock pot on the high heat setting to keep the spaghetti sauce warm because it could overcook the spaghetti sauce and burn it or cause it to stick to the crock pot.
Just place the crock pot setting on warm and add the spaghetti sauce and it will work just fine though.