It would be nice if the average soldier running alongside a tank could replace a dead Machine Gunner inside it and take control, as long as their not carrying any large objects, ie anything the tank crew can’t take such as rifles. I could be wrong but I believe most soldiers had the ability to use a Machine Gun even if they weren’t proficient with it (stat decrease). Failing that, this could be an option just for Machine Gunners outside of your tank crew but they preferably should be able to strap their gun to the outside of tank for later use.
When entering the vehicle with the average soldier you could make it so you automatically drop anything that’s too big (with a warning) or have the option to strap your rifle/rocket launcher to the outside of the tank.
Peace