It is definitely possible to change the working of this button, but to me it is working correctly (as described in the rules). The only point in which you can "add" temporary HP is in the case of the Battle Rager fighter build. On all other times the largest value is kept.

In my group I have a paladin that is frequently giving temporary HP to all characters and having it as "set" avoids accumulating temporary HP (which is incorrect by the rules). When I use the set button on all the character I don't have to do any math to check if the character receive all the HP. To illustrate my use case: character A has 3 temp hp, and B has 8, and C has 5 when Paladin grants 7 to all of them, I press set on each and I know I'll have the right temporary hitpoints (A and C will have 7 and B will have 8). I had an "add" semantic I would have to add 4 temp HP on A, 2 temp HP on C and skip B. This is a logic I avoid by using the "set" semantic (as described in the rules).

If you are using a battle rager, temporary HP will accumulate. But again having an "add" logic may not make sense, since you have to make sure you can or cannot accumulate temporary hit points (since only certain temporary HP accumulate). For the rager, I think you have to do the math (or let the player do it, and tell you how many temporary HP he has, and tell you).

However I would like to understand the use case for adding temporary HP. Are you using a battle rager? What would you do to take care of mass temporary granting powers from Paladins, Warlords, and others? Maybe two different logic may makes sense, but please illustrate you need for accumulating temporary HP.

PS: If you want a new feature add it to the issues tracker as a feature request.