I don't recommend putting frozen chicken breasts in a slow cooker. Frozen chicken breasts take more time to heat in a slow cooker than thawed chicken. Frozen chicken breasts may take too long to heat to a safe temperature. The best way to cook chicken breasts in the crock pot is simply to thaw the breasts first and then cook them up. In addition, if you brown the chicken breasts as per this recipe, you will want to thaw the chicken before putting it in a pan to brown.