The NICKNAMEID field in players.dbf refers to values in nicknames.dbf. Search for a nickname in nicknames.dbf, note the ID number, then edit a player's NICKNAMEID field in players.dbf accordingly. You can also add nicknames to nicknames.dbf with their own ID numbers, which you can then assign.
I'd also recommend using DB Commander rather than Excel; it's very easy to corrupt the databases when editing them with Excel.