Apply custom properties to specific configurations in the active part or assembly document.

With Api  set specific configuations

2012 SolidWorks API Help - Get Custom Properties of Referenced Part (VBA)

http://help.solidworks.com/2012/English/api/sldworksapi/Get_Custom_Properties_of_Referenced_Part_Example_vb.htm

Sub ll()
  Dim SwModel As ModelDoc2
  Set SwModel = Application.SldWorks.ActiveDoc
  Dim swCustProp As CustomPropertyManager
  Set swCustProp = SwModel.Extension.CustomPropertyManager("") ' .CustomPropertyManager("")
  swCustProp.Add "Mass", swCustomInfoType_e.swCustomInfoText, "MyValue"
  swCustProp.Set "Mass", "MyValue"

Stop

End Sub

SolidWorks Forums: API - How to change configuration name?

https://forum.solidworks.com/thread/78124?