Sorry if this is a bit of a lame question - but how do I reprogram HARD buttons? :/

I see I can customise and learn IR command for the soft buttons - but I'm not sure how to do hard buttons? For example, one of my devices (Showcenter 200) original remote has a 'bac'k button (reverse arrow icon). When I learnt this on the One, it came out as a soft button called 'back'.

Although this worked, the One has a reverse arrow as well. How do I map the IR code to a certain hard button?

This goes for my Logitech SqueezeBox as well. The original has a + ... and so does the One. How do I make the + on the One do the same thing as the + on the original?

Thanks!