http://ejinz.com/YourHome/Wiki.jsp?page=ACustomCheckinOfACADDocumentWithTheCreateOrAssociatePartsToDocsOptionToAWTPartTha

Using Wildfire 2, the user wishes to passively link a CAD Document to a WTPart
that already has an active link to separate the EPM Document through a custom
checkin with the Create or Associate Parts to Docs option enabled. 

Setup: 
-Disable auto-number generation for EPMDocuments and WTParts 
-create an IBA using the Attribute Definition Manager 
-modify the cadxhtmlui.ini (located at <WT_HOME>\codebase\com\ptc\windchill\cadx\cfg\default) 
property AutoAssociateNumberingParameter=<IBA> created in the Attribute Definition Manager> 
-create a new workspace not using the Wildfire Server Registry to implement the cadxhtmlui.ini modification 
-create a designation parameter in the CAD Documents with a name matching the newly created IBA, 
String type, and a value equal to the number of the WTPart for which the user
wishes to create an active association to the CAD Document. 

A custom checkin of the CAD Document with the Create or Associate Parts to Docs option 
selected to a WTPart that already has an active association to a different CAD Document 
will result in no association for the CAD Document or WTPart creation.