Ok go to "The Forge - Mod Development" forum, enter Unofficial Editing tools and download ItemEditor and BRF Edit, they're both on the frontpage. Download OSP Weapon pack.
Make a backup of your native folder and your save games folder located in My Documents.
Now extract the contents of the weapon pack into your native folder, DDS files to the Texture folder and BRF files to the Resource folder. This tutorial will add a sword, it's exactly the same as adding a polearm, or an axe, just it's a different model.
Alright, now that you've done those things, go into modules.ini and then to the bottom of the Load_Resource lines, add "Load_Module_Resource = OSP_Swords" there. Now open OSP_Swords.brf in the resource folder, find a sword and memorize it's name. In this tutorial, we will pretend the sword is called lui_sword.
The RRR swords have 2 meshes, and you have to merge them in wings3d, which I well get to after the tutorial to adding items.
Alright, now enter ItemEditor and press Open File, find item_kinds1 and open it. Now find an existing sword, if the sword is a 2 handed sword, find a 2 handed sword, if it's one handed, find a one handed sword. In this tutorial, the sword will be 2 handed. Alright, we find a 2 handed sword, like the Great_sword. Click the tabs above until you see "Insert", hold the cursor over insert until you see a new tab with "Insert end of the line", press that button and the great_sword will be cloned and the clone will be put at the end of the item_kinds1.
Browse to the end of the file and click your cloned Great_sword, change the ID to whatever the mesh in the BRF file was called, in this case it is "lui_sword", remember, it has to have itm_ infront of it, so "itm_lui_sword".
At Display Name 1 and 2, you choose the name for the sword, what it will be called in the game. Let's name it "Lui_Sword". For every space, a "_" is required.
Underneath the display names is another window, called "Mesh", click the existing mesh and rename it "lui_sword" (or whatever the mesh name is) without an itm_ this time.
Now you can adjust the damage of the sword, and the type of attack, pierce, blunt or cut. It's easy to find those. Save the file and exit. Now enter the game, and you have your new sword.
Alright, now for the RRR swords. To merge them, you will need a program called Wings3D, google it and you will find the program at their homepage, it's free.
Now that you have Wings3D, go into BRFEdit and find the RRR sword you want, say it's the "RRR_Agincourt_steel", export both files, Agincourt_steel1 and 2. They will now turn into .obj files. Place them in the Wings3D folder, now open wings3d and press "Import", import both Agincourt_steel1 and 2. When you've done that, press CTRL+A and all polys (I think that's the name for it) will be selected, now press "Export Selected" and name it. They will now be merged into one file, one sword. Move the new file into your resource folder. Open BRFEdit and press "Import" and import the new file. Now add it just like you added the Lui_sword, of course this time the ID would be what you named the file, and the mesh name.
Hope I helped, good luck.
Oh and, if you get any errors, report them here.