That's a really good idea. Some optics like the Bushnell spotting scopes are a pain in the butt even with Eye pro. It would be nice if something existed.
A simple negative lens element behind the ocular will increase eye relief, but at the same time decrease magnification. It's the reversion of the principle behind those "boosters" (positive lenses) that will increase maginification at the expense of eye relief.
There's no way to just get additional eye relief. No free lunch to be had here.
I can increase the magnification of my scope, by simply taking my glasses off! Leupy tactical scope, not sure if it's because of the tint change or what, but it does.