Upgrading a shield

I've got a mighty fine Shadow Shield here which is gold and has the stats:
Armor 16
Damage reduction of 5 (Chance 20%) (It's not an inscription)
Req 10 Str

I want to add a shield handle of fortitude to it, which should be possible. (At least I think so) But I can't, it doesn't let me add it to the shield, can anybody tell me why or how to solve this? Thanks.

The only shields that take any mods are the inscribable ones (shield and offhand mods were not added until inscriptions were). If that -5(20%) is an inherent mod, that means the shield is stuck how it dropped.

Okay thanks, then I'll tell my friend he's screwed :P