can a basic weapon with melle attachment be used one handed in mellee and would it keep qualities like accurate?

In theory, it can indeed be used one handed. Assuming your GM is happy with the idea of a bayonet being used in one hand.

But no: It would not gain the benefits of the weapon. That's just...bizarre. Put a knife on a plasma gun and making it volatile?! Or on a sniper rifle and making it accurate?! No.

I ment would a weapon say a satlker pattern boltgun still be accurate if fired one handed with a mellee attachment

That should still be the case, however remember you must still aim.

With the melee attachment though, some GMs (read me), have issue with using the actual melee weapon one handed, primarily as a leverage thing, so check with yours on that detail.

My GM allows 1-handed use of the melee attachments, but rules that they are Unwieldy when so used.

thanks. I am the gm for the game just looking for feedback to help me make my decsion. One of my players is thinking of going down that route with his assault marine. He plays him quite cautious waiting for the enemy to approach the rest of the squad before going hand to hand and is looking for some long range options without loosing access to his off hand attack.

I'd personally not be keen on a player for aiming and firing a sniper-weapon one-handed and gaining any 'accurate' bonus damage, but that's just personal taste, because it seems a bit silly. "Yeah, this is a precision balanced sniper rifle, with a free-floating metal barrel, x20 scope, and accurised ammunition. It can kill a man up to a mile away, and has fully-custom grips and stock" "So why are you firing it one-handed?"