I have a fostech mach 2 rail. It doesn't have much picatinny room and my red dot occupies most of that space. I would like to buy and attatch a magnifier to use with it. Anyways my question is how do I add a picatinny rail section to this? Or is there another way to accomplish this?
Yankee Hill Machine makes a 6" rail extender that mounts to your existing rail. It will raise the surface 1/2", so you may need a 1/2 riser for your magnifier. You also may run out of room near the charging handle for your BUIS. It also will make it hard to co-witness your sights. Here is a link, hope the mods don't count it as spam. Amazon.com: Yankee Hill Machine Rail Extension, 6" (YHM-9473): Sports & Outdoors
There are a couple of other manufacturers, but all of them will raise the surface of the rail.
You could consider just getting another optic. I have a Vortec Spitfire 3x prism with ectched reticles it is an older model, they now have a Gen 2, spitfire prism in a 3x and 5x, eliminating the need for a magnifier that should meet your needs without extensions , new hand guards, and duct tape....just saying.
This is just an opinion, but I think that sight has too much going on behind the actual optic plane to do much with it regarding a magnifier. You might be able to move it forward and come up with a compromise, but it's going to be cobbled together for lack of a better description. The Eotechs have the batteries and smarts forward of the optics to make room for a magnifier. Your Sightmark has a lot going on out back toward the rear of the rifle. Personally, I wouldn't be a happy with a holographic sight in front of a magnifier anyway, but if you've tried it and like it then you may want to rethink the primary optic. I've used an optical magnifier while sighting in a Primary Arms red dot for a friend and I found that I shot more accurately with the magnifier out of the way. Apparently, it worked because he was happy with the final setup. By the way, welcome to the forum!
EDIT: I just noticed that the quick disconnect is in the middle of the optic. It looks like you have several slots more forward that you can go with the mounting possibly if that's true. Still, I think it would be tight.