Yes you can place frozen chicken in the crock pot before thawing it out.
The chicken will just take a bit longer to cook in the crock pot when frozen but the chicken will cook just fine in the crock pot.
It can take around 1 to 4 hours to fully cook the frozen chicken in the crock pot but the good thing about a crock pot is that the crock pot will continue to cook the chicken or any other food while you're away from the house and doing something else.
I always just remove the frozen chicken my freezer and place it in the crock pot and add some water to the crock pot.
After that I set the crock pot on high and after I come home 8 hours later the chicken is fully done and ready to eat.