Blacksmith Rupio:
First, you can't use the old Soul Crystals anymore. To bestow the new special abilities, you need the new Soul Crystals. You can get them by defeating boss or raid monsters.
Well, it's better to try it. Here's a Practice StormBringer and a Practice Soul Crystal - Stage 1. Use this Practice Soul Crystal to bestow a special option onto the weapon.
You need 177 Gemstones (C-grade), but for now, you should try out by purchasing them. I'll give you just as many as a reward after you succeed. If you want, you can try it on another weapon, but I advise you against it, since the Practice Soul Crystal isn't as powerful.