It appears to be the item you need to upgrade some old QL200 RK weapons into newer (some quite neat) types.
The Xmeneth Plasmaprojector is one of these (there are about 8), but appears to be the only one dropping in team missions thus far - so its probably not the correct way to obtain these weapons.
Only clue I have about the 'Viral Weapon Upgrade' kit is that... it has a little vial as an icon.
Dropped from Brigadier General on ship.
I got a ql 74 or so off of a general invasion mob. so far the only one my org has found.
I got a QL200 from the General on the ground, and combined it with a QL200 flamethrower. Made the Premium Merren Flamethrower. Quite nice, I think. Only reqs are AR and burst, and has pretty fast recharge rate. Damage is 150-450 (450) and rates are 2.5s/1.5s
Not exactly a Coop or a Peren, but not bad.
is it possible to use the weapon upgrade multiple times?
Negative, one time use only.
And also, this is the list of some of the weapon types that can be converted:
Also bear in mind that the viral upgrade can be combined into other weapons. When placing the upgrade into the tradeskill kit as the source, 107 results come up, so there are a lot more. Perhaps it is only the other names for lower QL weapons (such as Balanced Flamethrower for the 121-140 range).
I have no idea about what the requirements are for the weapon versus the upgrade, nor do I know which is the final determinor for the QL of the weapon (I will assume it is the upgrade, but I cannot be certain).
Premium Xnemth Plasmaprojector dropped from a RK mission boss, so its not made by tradeskills.
Viral Weapon Upgrade + Premium Supernova Mk VI (http://aodb.info/showitem.aspx?AOID=122595) = Premium Kaetans Supernova (http://aodb.info/showitem.aspx?AOID=248577)
That one I can confirm, havent tried any other weapons.
Edit; 1k Weapon smithing req