With Forest Pro you can create your own libraries, using XFrog plants or other. As you pointed out, this feature is disabled in Forest Lite. But in any case, it is responsibility of the user to prepare the max files, materials and create the libraries. This is not an automatic process.
We are working to provide ready-to-use XFrog catalogs, in the same way that we offer HQ Catalog. They should be ready no long after the release of Forest 4, which is now under development.
Anyway, should not be problems using the XFrog plants manually: just merge the plants objects into the scene, and assign them to Forest (in the Geometry List). You also have the Optimize Material tool, to convert the XFrog materials to VRay/Mental Ray. This procedure works both for Forest Lite and Pro.