I just installed Allplan 2018 and check that the Xref still do not upload into the BIMplus model. If you have a project with lots of repeating elements and you want to work with BIMplus, you have two options:
1 Give up the excellent tool of external references, and manually copy the changes to multiple files.
2 Upload Xrefs to BIMplus as an IFC model, renouncing the advantages of file organization in Allplan
I find it hard to understand that Xrefs are correctly exported as IFC and can not be uploaded to BIM plus