If they're just mesh/texture replacements and have the exact same name/path of the originals, then no .esp should be required. If not, then you'll have to make an .esp with the Creation Kit and edit the objects you want to point to your new files. The meshes and textures are never stored/created to .esp. They can be kept as loose files or bundled in a .bsa, which is preferred if you are to distribute the mod to users and you have a .esp to go with it. I believe the CK has a built in tool for creating the .bsa, but there are other/better alternatives such as Archive.exe (also comes with the CK, but as a separate program) and some 3rd party tools. I'm not too familiar with mod organizer though so can't help you with that part.