Frozen chicken vs thawed chicken in the crockpot

You could cook it for less time, but it equally wouldn't make much difference if you just went for the full 6-7 hours, especially if you are using a forgiving cut like thigh.

I would, incidentally, be very wary about putting frozen chicken in a slow cooker. At the low temperatures a slow cooker uses, there is a good chance the chicken will be sitting in the 'danger zone' for longer than 2 hours, increasing the chances of food poisoning drastically.

The only time I would consider cooking frozen food in a slow cooker directly is if it is cooked in a liquid bath - i.e., a soup, stew, or similar food where the frozen foods are fully immersed. Additionally, the food would have to be reasonably thin; a chicken thigh would be fine, for example, but a full chicken is likely too thick, and might take too much time even immersed. I would also only cook on high - so that there is at least a bit of simmering/bubbling to help the liquid move around a bit. See the USDA FSIS's opinion on the matter for more details.

However, if that were the case (that it were pieces of chicken, not a whole chicken, and fully immersed in a liquid), I would expect it to not add a whole lot of time to the cook time to be frozen. Thawing chicken pieces in boiling water takes a matter of minutes - maybe a half hour, at most - and so would not significantly effect the time you are talking about. If you've ever used even 'cold water' defrost, you know that moving water defrosts food very quickly. Moving hot water is even faster.

This is similar to cooking sous-vide, by the way. That is considered a safe way to cook frozen food, subject to certain size restrictions similar to above, because it is immersed in moving liquid. Most sous-vide recipes suggest adding about 20-30 minutes for frozen product, depending on the precise details. In my personal experience, it's even less; cooking frozen chicken thighs bone-in, for example, took only 15 minutes more to reach temp than raw unfrozen. That's cooking at 145°F, significantly lower than your slow cooker likely cooks at. Modernist Cooking Made Easy recommends 15 to 30 minutes extra for steaks, which I agree with as well.

But, again, I would not trust a recipe that asked for a frozen whole chicken or chicken parts and wasn't cooking it in liquid: I would go find another recipe.