With how weapon mods currently work is it possible to modify say a mosin nagant or similar weapon with an internal magazine to instead accept detachable magazines without the need of making an entirely new weapon?
Technically yes. I was more of thinking along the lines of trench mags from ww1 but yeah.
Answer: as the game is currently programmed it is not possible to convert a fixed mag to detachable mag weapon. So if you had the idea of adding trench magazines or otherwise you will have to make it a crafting based conversion.