Sure, you love the game so much that you wanted to improve it by creating new content or improving on the existing stuff, but then you get tempted to grab something from another game, converting it and then incorporate it in. As if it wasn’t enough, you want to bring the mod out for public consumption.
Simple? No. If you rip content, you could step on an intellectual property landmine, because a mesh, mode, a texture, or even a sound file from another game or a mod author could be protected by copyright.
I found this out after I saw that a Skyrim mod author based in Hong Kong was banned — for the third time — by a well-known mod site for what was considered the use of copyrighted content, which brought me to the conclusion that the site is akin to Youtube’s policy: produce your own original content, not ripping off someone’s stuff unless you have asked permission and granted so for modification/distribution with due credit.
If you don’t like their policy on IP and it gives you a chilling effect, then you’ll have to host your files elsewhere. That’s it.