Your server has a built-in FastDL download server. When a player on OpenMoHAA joins without one of your custom maps or mods, the game downloads the .pk3 file from it automatically. You don't need to set anything up for this.
Note
Only OpenMoHAA can download files from a server, and players need downloads turned on: open the console with ~ and type seta cl_allowDownload 1. Players on the original game need to get your custom .pk3 files before they join.