Actually there are LPVO style scopes just for marksman but a 1x-6x LPVO scope should be available to all rifles at a greater cost.
It could have 3x magnifications settings like 1x-3x-6x and the option to assign different buttons to increase or decrease magnification or use the same button to do that.
Some people may think that if you add a LPVO to any AR it will work like an DMR but it will not. DMR and Sniper rifles will always be more accurate at long ranges and have more power in the bullet, so its not difficult to balance different rifles in the game.