Global Search and Replace might work. Back up your data first, and select Create a report of records changed.
There are utilities that can add a note or source to gedcom files. AddSource and AddNote have been used successfully by some people. (the Windows version of AddNote seems buggy, but I have used the DOS version successfully.) The free Legacy6 software can add a source to all records.
I considered the possibility of editing the gedcom in a text editor but I don't think it is workable.
Try a search on Google for gedcom utilities.
Does anyone have other suggestions?
Legacy6 might be the best bet. legacyfamilytree.com