Haha, i just modded it today. Yes there is.

1.Open export_descr_building and look for highways

You shd see something like this
highways requires factions { roman, }

2. Add ur faction

say u wanna add parthia then....

highways requires factions { roman, parthia, }

remember the , after parthia. it is vital otherwise ur game will crash to desktop

3. Then u need to add the building card for highways and for that u have to download vercingterox's pak extractor (go under downloads to find for it)

reply once u have

goodluck