The game mechanic requires a fatal wound to do healing. Healing specifically removes one fatal wound and also gives back a number of HP, which you can set using mods. There are several mods that introduce different healing items, that heal from 0 HP (so just removes fatal wounds to stop bleeding) to lots of HP.
In theory, you can create a medikit that restores 200 HP per fatal wound and that would be similar to healing not requiring fatal wounds. It wouldn't allow quite the same gameplay (you heal all in one go instead of by repeatedly allying small heals) but it would allow you to heal up a soldier.
That's kinda weird though. A field medic can bring you back to shape mid-battle over what is at most a few hours for the whole battle, but if you are wounded when you get back at the base it takes 3 weeks to heal up? I think the current system which forces some recovery time makes more sense, albeit with a place for improved medikits that heal more HP per fatal wound (at the cost of research, manufacturing materials and/or TUs).