I'm not so sure about adding effects. I don't think players shod be able to create special weapons out of mundane ones. It would spoil the fun of finding the special weapons.maybe something like a skill (Apothecary) that allows you to either add an status effect of raise the effects of a status effect on a weapon.
A way to raise the status effects is a great idea though. One option would be a skill. Another possibility is that you would have to take the weapon to a NPC somewhere, and they would require some rare ingredients and/or lots of gold to go it. Maybe a given NPC could only raise one or two effects, so you would have to find the right one, then go and get whatever they needed.