You need 3D Object Converter (paid, cracked often doesn't work well).
Extract the weapon file (whatever file extension Aion uses) and texture. If they are not the same name, rename them so their names match (eg. Weapon1.obj and Weapon1.png for example).
Open the weapon file with 3D Object Converter and press the button to show the texture (not really neccessary, but recommended as it makes editing easier).
You can save the file as is by clicking on Save As and selecting Shaiya Online .3do, but it will 99.9% not work right out of the box.
Go to Tools and select Geometry transormation. There you will have to play around with size and orientation to make the weapons appear properly sized and oriented. I could tell you how, but you will have to figure it out by yourself to learn it properly.
If you want to convert, for example, a 2handed sword, you can use any 2handed sword from the shaiya client as reference to see how it is oriented when you first open it with 3DOC, then try to replicate the exact same orientation of your custom weapon.
Once you successfully converted 1 or 2 weapons you'll see how easy it is.
With this tool, you can only convert 3d models that are static (no bones), since 3d models with bones are a bit more complicated and I haven't seen anyone converting armors from different games to shaiya yet. This means you can also convert helmets and capes.
The screenshots are dead, but you can look at some of my files I decided to release in the topic "Custom weapons and other stuff" in my signature.