Can I add Avails for multiple territories?

Yes. Under Available Rights, choose "Only Specific Countries/Regions" in order to enter specific available countries/regions for your Listing.

You can similarly do this in the Sold Rights setting. Here you would exclude the territories that are sold (Available Rights are inclusive). You choose which of these two routes is easier for you; BUT make sure they do not conflict (a territory could hide in a Custom Region, and fall into both options!).