Okay, now I'll address your other two questions.
Mounting a bayonet on a magazine tube extension is probably not wise, because that tube was not built to hold a bayonet sturdily enough for you to stab anything with it or apply such for to it, it's would likely cause it to bend or break, which would obviously mess up the functioning. Mounting it side-ways on the barrel seems more do-able, although it would look terrible, and really, why on earth would you need a bayonet on a SuperNova? Shotguns with bayonets have that added extension that infuses into the heat shield, like on a Trench shotgun. If such a devise could be modified to mount on a SuperNova (which I'm not too sure about), you might be able to pull it off. As for how much it would cost to mount bayonet lugs or to customly fit that Trench mount to it, that's really depending on which gunsmith you see, and if they'd even say it's worth doing.
As for modifying a mag-tube shotgun to take detachable magazines, I imagine that would take professional gunsmithing or some sort of conversion kit.
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