is it possible to create a MP mod for GTA V
Unfortunately, you cannot and are not allowed to modify any aspect of GTAV and attempt to access its online features.
Online modding is highly discouraged and you will be banned if you somehow even successfully get online with modified files.
I suggest you stick to a clean install of GTAV when accessing online as you will be banned from R*.
Please, stick to using single player mods only. There are plenty of mods on this site that will allow you to mimic as much online exclusive content in the single player.