Hi I am making a chocolate pumpkin pie and I cant get pumpkin, could I use frozen mixed berries instead

Hi Brid! What HalfPint said—frozen mixed berries have a lot more water than does pumpkin, which will result in a thin/runny filling. Do you have access to a butternut or kabocha squash instead? Or, perhaps you could just make a plain chocolate silk
pie: https://food52.com/recipes/76609-french-silk-pie-with-chocolate-coffee-grounds-crust ?

No. Berries don't have any fiber. You would have a runny chocolate soup that will never set. Try making this: https://www.foodnetwork.com/recipes/ina-garten/frozen-berries-with-hot-white-chocolate-recipe-1942823

It works with dark or milk chocolate too.