I think you could feed green peppers that were frozen. Most veggies like green beans, peas, carrots, etc., must be blanched in order to freeze and properly preserve them. So they are cooked in boiling water for about 2 minutes, then frozen. However, green peppers don’t need to be blanched. They are like fruit and can simply be cleaned, sliced, and frozen.
I feed frozen corn husks to my guinea pig. I get them in the grocery stores where they husk the fresh corn. I remove the outer green parts in case there is any pesticide or other residue. Then I wash and tear the husks in strips, lay them out on a cookie sheet and freeze them. Once frozen, I put them in freezer bags and take out what I need for a day or two. I have also frozen watermelon cubes which were a big hit. They stay soft enough that she can eat them frozen so it was a nice treat during the winter.