Yes, there is. Several scenario are supported:
 select all or part of the layout you're editing with Scene Builder 2 and drag and drop it on the Library panel
 use the Library menu (right to the Search area in Library panel) and select "Import JAR/FXML file"
 directly drop on the Library panel one or more jar and fxml files from your favorite file browser
The import of a jar file will make an import dialog window coming up from where you can select a subset of jar's content.
Let us know how it does.
Well, to be sure there isn't some setup issue or the like could you give a try with a jar like http://oss.sonatype.org/content/repositories/snapshots/org/jfxtras/jfxtras-labs/8.0-r1-SNAPSHOT/jfxtras-labs-8.0-r1-2013… ?
I anticipate that even with the jar file I referred you'll continue to get a dialog where the list of items is empty, isn't it ?
This is a known issue and the sole remedy is for now to wait for the next Developer Preview drop of Scene Builder 2.0 (coming soon) !
Thanks for your interest in Scene Builder and share with us your experience when you'll have the next build installed.
The jar I referred doesn't given a proper user experience with Scene Builder 2.0 b07, which is the one I guess you're using: in the Import dialog the list of items is empty as it should not. But I agree that if you click anyway on Import button then the items, all of them, are added into the Custom part of the Library. I assume this is what you mean by "Your jar works".
With the next Scene Builder build the list in Import dialog will not be empty, but this isn't what bothers you most for now: did I understand correctly ?
Why nothing gets imported at all with your jar may require further investigation: please log a JIRA here https://javafx-jira.kenai.com/browse/DTL