That isn't hard to do, as long as you stick to certain naming conventions, although at the moment the shading layer would be a problem as it is set to use multiple shade brushes for every tree. So unless your tree outlines were very similar in shape and size it would look pretty bad. It's on my todo list of little things to fix before I sign off on this but I'm still unsure as the best way to go about it, naming-conventionwise. I want to keep the ability to have multiple shade brushes per tree, but then when you add in multiple trees the naming gets more awkward. I guess Shade1_1, Shade1_2 etc. like I suggested several pages pack is the way to go.
Of course, you don't need to use the shading layer if you don't want to.
I also need to make a small addition so that brush sets can be changed on the fly. It can already do it except I haven't added the option to the UI yet.