It's not easy to replicate the actual Haas, or Precedence effect [the time-delay & phase differences normally perceived by 2 ears] in headphones, even for non-hearing-impaired listeners.
That becomes far more difficult to do for only one side of the stereo field.
You could delay one channel by a set amount [I haven't checked the actual delay required to simulate this] but for a true '3D' image, the delay for a sound from the front is not the same as from 90˚ to one side. The delay would have to actively compare where the original sound source ought to be in the 360˚ horizontal plane & adjust accordingly.
It could possibly be done at source, but I don't see how it would be possible from a simple ready-processed stereo soundfield.
Edit:
The simplest solution may be to just sum the 2 sides to mono, with no delay. Then when any given sound is 'loudest' it is either directly in front or directly behind your character. There would be no way to tell which of those even by turning round, without Haas to help you, unless the game employs some equalisation changes depending on 'front' or 'rear' for non-hearing-impared listeners - as, of course, in simple stereo, Haas won't properly be assisting them either.