If you mean Hotshot packs, a long-las can already use those so no modification would be required. As for what would be required to "upgrade" a gun, that's heresy by Imperium standards, so normally Trade (Armourer) wouldn't apply, since that's for building from STCs that are well-known and discovered. Such a thing would be unthinkable to any good, clean Imperial citizen.
Screw that though, you're a Rogue Trader and you recruit Hereteks by the dozens - good thing too since they kill themselves off in blasphemous experiments at about that rate. First have them make a Trade Test to build a Long-Las, and then a second one to build the Hellgun (still for the same weapon) to prove that they understand the principles of both. Then combine the two into one ridiculous difficulty (which could put it below -60), and for every Shift of success they scored on their Trade tests previously, lower the difficulty by 5. As long as the cumulative modifier for the final Trade ends up above -60, they can make one last test to forge the weapons together.
As for what it should be, "Accurate" and "Semi/Full-Auto" never made a lot of sense to me. I would just allow your players to use the damage of a Hellgun and leave the remainder of the weapon stats as a longlas. Then bring the Mechanicus around eager to know where they found the STC for this marvelous new weapon he's been producing.