IIRC, those knobs have female threads, so the bolt handle has to be ground down and threaded. I don't recall the thread size, but getting a die and die handle should be no problem. (Brownells has both, and many hardware stores have dies for common thread sizes.)
Whether it is a DIY project depends on what equipment you have and how familiar you are with metal work. You will have to grind the end of the bolt handle to the right size, thread it, and then screw on the handle. You can choose whether to cut the original knob off or grind it down to give an even longer handle. Not a big job or beyond the capability of the amateur with a little experience, but not real easy either.
But it would be fairly easy to mess up the bolt handle, and then the job would become bigger. If you are not comfortable with metal work, taking the job to a gunsmith might be the best way to go. (And if HE messes up, HE has to make it right!)
Jim