Is it possible to increase the number of creature slots from 7 to 10 in castles/for heroes?

This is a concept for the third tier of recruitable upgrades in a town. I wrote it for testing; the third upgrade for the peasant building allows them to be upgraded to improved gremlins for testing purposes. Download the test file Thank you, Waterfall, for creating such a detailed video demonstrating the feature and explaining how it was implemented.