Just drop the map file (ex: occm_inhell.pk4) in your mod folder (ex: for opencoop, ...Doom 3/opencoop/), the mod should detect the map automatically
For OpenCoop, if you wanna host a listen server, you must select the oc_customs map type when you're going to host
In order to host Doom 3: ROE maps (campaign and also custom maps that require ROE) you must edit your launcher and add: "+set fs_game_base d3xp"
+ Non-steam users:
Create a new Doom 3 link from the original Doom3.exe and do the next.
For Opencoop: edit the launcher options and add at the end +set fs_game_base d3xp +set fs_game opencoop
For LMS4: edit the launcher options and add at the end +set fs_game_base d3xp +set fs_game lms4
Read "2. I can't host Doom 3: ROE campaign (OpenCoop or LMS4)"