Well, Xfrog plants typically either come in the Xfrog format (and you'd need Xfrog to convert them to something else) or in a regular 3d format (.3ds, .obj etc). You can't use either of those as Vue plants. The Vue plants (the ones with "infinite" variations) are specific to Vue and only Vue can make them.

In other words, you can't use Xfrog plants as regular Vue plants. You need to import them the way you'd import any other object. But, as far as I know, the PLE can't import any kind of object.