Rules wise it would be no different than a hooked spear, which are perfectly legal (though rare).
As far as functionality goes... I really don't see that buying you anything. Sure, now you can hook people's shields and move it around, but with a dagger like that you have to be in close enough anyway that if you
didn't have the hook on it it would be slim enough that you could just stab around behind their shield anyway. I suppose you could use it as an off-hand parrying weapon/shield hook, but then why not go with an axe (with a stabbing tip, if you want), so you get more reach and more attacking options?
Anyway, it's always worth a shot so good luck. Worst case scenario, you'll get some good knowledge for making a shield hook on a spear later on
.
WikifiedSquire to Sir Kyrian; Commander, Clan of the Hydra
"There is only one appropriate attack for all polearms, and that is the thrust."Antonio Manciolino, Opera Nova, 1531