Short answer, NO. Long answer below.
All the models used are stored in the following location:
<loadpoint>Creo 3.0\F000\Common Files\afx\parts
You can drill down deeper to see specific fasteners. For example, Holokrome screws are here:
<loadpoint>Creo 3.0\F000\Common Files\afx\parts\screws_and_pins\screws\HoloKrome
From a cursory look, it would seem that a part without a family table is loaded, then the family table itself is created from the .dat file, then finally the specific family table instance is saved as a stand alone part for use in your assembly. This whole technique is NOT going to play well with a PDM system.
I opened one of the parts and added my own parameters. Unfortunately, as soon as you attempt to place a new fastener with the modified part you get a pop up saying you can't use it.
There seems to be some pretty touchy file checking that is making sure you haven't changed anything.
One more weird fact, I found it fascinating that the part I modified (hk-shcs-unrc.prt) was last saved in 2004 using Pro/e version 2001. Pretty old parts.