Weapons have to be rigged to a pair of arms and be bone driven. What model program are you using?
Open up the gamecore\guns folder and take a look at some of the weapon under it. When you open a weapon folder you will see 3 x files. One called hud, one called Vweap, and one called the name of the weapon.
The one that is the name of the weapon is the weapon pickup model, the one called Hud is the characters arms and the weapon when chosen in the engine, The one called vweap, is for the NPC when the weapon is assigned to them.
The gunspec.txt file lists the animation ranges and assigns the sounds and other airmode stuff for scope zoom and blocking and so fourth.
So if you have all three models of your sword ready then you could copy a weapons folder, rename it, replace the models and textures and try it out.
Will take some practice, especially if you have to rig the arms and weapons, but after time will become common place for you.
Good Luck.