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

I think you should define your new angel in the building as the second, not the first creature. Maybe then it will appear in the second slot. But there's no guarantee. In any case, with just one new creature, it doesn't look very good. If there are two upgrades, then you'd want two new upgrade levels as well. Ideally, you should just take the core of the Necropolis faction and add as many creatures as you want. Or, if it's the vanilla version, there's still the Ettin available.

Added 3 minutes ago

(First of all, upgrading the Dweller to the third level will remove the ability to hire regular angels, and in certain situations, this could be a problem.)

Well, at least this problem has been solved.

Added 5 minutes ago

As far as I remember, I found all these conversion values in some "mechanical" xdb file, and it's very possible that it contained combinations of upgrades and alt-upgrades for each city level!

If we could find that, it would be great. We could fix the last bug.