add specific number instead of exploding dice

Hi, everywhere I read about exploding dice. In my RPG the extra-damage is calculated much easier, but I don't find a way to create the right formula. When I crit (max. roll), then I just add i.e. 2 dmg and not an complete extra-die. So when I roll a 6 on a d6, it's 6+2=8 dmg, otherwise it's 1 to 5 dmg. So how can I add my +2 only to the maximum-roll? Most likely it's trivial, but I don't get it :-( Thanks for your help.

You are likely going to need to use a rollable table to mimic what is effectively a d6 with no 6 but an 8 instead. 6 entries weighted: 1 - weight 1 2 - weight 1 3 - weight 1 4 - weight 1 5 - weight 1 8 - weight 1 You'd then call it by rolling 1t[speciald6] (or whatever you call the table)