Note: You may notice, when opening the standard part, you will not be able to access any command so not even vba macros. You can't issue the vba macro directly from part only mode.
The way to run this macro is:
 
1. "save copy as" of standard part
2. from the assembly, open the visual basic editor (tools\macro\visual basic editor or ALT F11)
3. Copy the code below into module1
Sub UNlockFile()
  ThisApplication.ActiveDocument.DisabledCommandTypes = 0
End Sub
 
4. open the standard part (part only mode) 
5. switch to VBA Editor and run macro by pushing the "run macro button"
6. Done. Now you can access any feature, adding custom properties etc.