In Integrity Lifecycle Manager 10.6 this is already the case. If you add a relationship in the GUI, by default it will add that relationship to the end of the existing relationship set. If you add from the command line using im editissue --addfieldvalues=<RelationshipName>=<ItemIDToRelate> <ThisItemID> it again appends the related item to the end of the list of items in the relationship list.
Does that help, or do you need additional information?
Kind Regards, Kael Lizak
Senior Technical Support Engineer PTC Integrity Lifecycle Manager