+1 on the RTG offerings above. Robert is a class-act with whom I have done business for many years. He'll take good care of you.
HOWEVER...if you want a Harris or swivel stud type setup for a similar bipod, there are way to do it fairly easily assuming you have the traditional "wide" HK91 forearm and NOT the slim metal one. If that'd be helpful to you, let me know and I'll post the instructions for you. Its a 15-min job max with the proper Harris adapter and tools.
As for mounting other "accessory rails"...check out some of the aftermarket forearms from companies like JLD/PTR, B&T, etc. that are already railed. There isn't enough room in the standard handguards to allow for mounting other rails.