If it's just a script with no datablocks,
either discoverFile("path/to/file.zip"); or setModPaths(getModPaths()); (latter one probably takes longer in most cases but might be less cumbersome to type). This only applies if it's in a .zip file though, if it's just in a folder this isn't necessary.
and then exec("path/to/script.cs"); (e.g. exec("Add-Ons/Script_Mouse/server.cs"); )
If it has datablocks, then you run into a few more issues. It's generally a better idea to just restart the server in that case, as datablocks aren't meant to be modified like that. Though there are workarounds.