I do it frozen all the time. There is no issue with temps as it won't stay at the "perfect" temp to grow bacteria long enough for it to be a problem. Crock pots heat up pretty fast. I've also cooked frozen chicken, pork chops, etc. I'm very careful with food prep, but a frozen roast in a crock pot is fine.
I sprinkle salt, pepper, garlic and a little gravy flour on top add onion and water. Then the last couple of hours I put in potatoes and carrots. I then use the drippings to make gravy. I've been doing it this way for 30 years and it always turns out great, not to mention safe.