Yes, FBReader does support prc. You can add this extension to the list in the file
Code:
\Programs\_fbreader\_ctbconf\etc\gconf\schemas\fbreader.schemas
and then re-run file associations.
Or as a simple workaround you can pack prc files into zip archive and add 'zip' extension. This would save some space in a card by the way.
Works for me either way.