Maighstir: Technically, you
can add funds to your "GOG wallet", but it costs a little bit more than the funds you're adding. First, you'll have to go to a country where the regional price is as high as possible (since the US of A doesn't count -- I would guess Australia or New Zealand are the best bets, European countries work as well, but you probably won't get as much credit from there as from Down-under or Middle Earth), then you purchase any game which is regionally priced and gives store credit (I don't know if different games give different percentages as credit, they shouldn't, but there may be small differences, I suggest you compare the whole catalogue for the best ROI). And you continue purchasing it, not using any of your accumulated store credit in said purchases, until you have as much credit as you want to have.
There you go, easy as pie.