Using external stub types as property types

Hi,

imagine having successfully imported an external jar. A freshman's idea could be to use one of those imported types as a concept's property or child type, e.g.

concept C extends BaseConcept                                                                                                                                                                                         
                  implements INamedConcept                                                                                                                                                                                    
                                                                                                                                                                                                                                  
... 
 
properties: 
p1: ExternalTypeA
 
children: 
c1: ExternalTypeB[0..n] 
               

But this isn't possible by default. What would you suggest to attach objects of an external type to concepts?
Any other ideas which suit the conceptional approach of MPS any better?

Regards
Kai