I don't know, how? Wait how do I get the punch line?
If your transmission is separate from the engine, you can swap it out for a unit so equipped. Look for some trike.
Why do you want to do this? There's a reason bikes don't come with reverse. It's a dangerous idea. Trikes have stability so it's not so dangerous.
If the bike is too heavy, get something smaller. Learn to plan your parking so that you pull out forward.
