Could you explain what you mean?
Sketch will allow you to override every nested symbol, including the text symbols. So all you need to do is override the text string on the button symbol, then resize the button until the text no longer wraps. This will ensure the padding is equal on both the left and the right side.
If you want a button with bigger or smaller text, you will have to make separate symbols for those. That’s because sketch will not allow you to override the text symbol with bigger or smaller fonts because the art boards are not the same size. So in my base kit, I have three sizes of buttons. But I would much rather prefer three symbols than 30!