xmodmap -e 'keycode 65 = space underscore'
Properly typing an underscore should utilize two hands and shouldn't require a stretch at all.
Either way, reaching the + key on QWERTY-102 is objectively more difficult than the underscore [uses the pinky, is farther away than the underscore from homerow], and also requires a shift-modifier. I use + far more than _ while I'm coding. (And I'm currently writing most of my code in Go, so I use _ quite a bit since it's a blank identifier.)
Maybe I just have big hands, but it honestly doesn't impede my typing at all to add an underscore to the text.