Adding Maps to Insurgency is straight forward. Do the following:
Download the map
Drop the files (*.bsp, *.nav and *.txt) into the maps folder.
Maps folder location: C:\Program Files (x86)\Steam\SteamApps\common\insurgency2\insurgency\maps
Example:
I downloaded the map called DOW.
I ended up with DOW.zip
I extracted the file and ended up with a folder containing 3 files:
1. DOW.bsp
2. DOW.nav
3. DOW.txt
I copied and pasted them into C:\Program Files (x86)\Steam\SteamApps\common\insurgency2\insurgency\maps
Job done