For others arriving at this looking to upgrade without completing the game first:
Mass Effect 3 Gibbed save editor.
Go to raw, player, change new game count from 0 to 1. Weapon X upgrades unlocked.
He later posted a correction which I overlooked:
My fault, was typing that advice out of memory. It's actually "raw/plot/plot".
To change weapon mods past level 7 without finding the weapon itself go to "raw / plot / player variables" then just look up the weapons you want to change, to find the items easier just focus on the numbers with 7.
Credits go to LeonZweihander on gamefaqs.com