Fred, I include the witnesses to any event, as part of the text in the Source, as in this context:
COMMENT:Informant was Sarah (SMITH) BROWN, mother of Anytown;witnesses were Dr. Freeman, Medical Attendant and Mrs. Morgan, nurse;no exact place of birth, other than Anytown was recorded;registered on 30 Sep 1879.
If you are looking for some way to be able to conduct a "search" for witnesses names, it could only be done, I think, if they were part of your family file and entered as such. You would then have to make reference in their "Notes" to the effect that they were a witness to the birth, marriage, death of a particular relative. Then of course you would have to have the reverse Note recorded in that relatives Note section. If they are not part of your family records in anyway it would be difficult to cross reference them in PAF, but you could do this in a program like CLOOZ, CUSTODIAN, BYGONES, or create a database in Excel or Access, perhaps.