I tried to use the lite-xl gui to add the keypad + key to zoom, for some reason while the key gets recognized it doesn't actually save the changes.

Editing the configuration file directly to add the keybind like this:

      ["scale:decrease"] = {
        [1] = "ctrl+-",
        [2] = "ctrl+wheeldown",
        [3] = "ctrl+keypad -"
      },
      ["scale:increase"] = {
        [1] = "ctrl+=",
        [2] = "ctrl+wheelup",
        [3] = "ctrl+keypad +"
      },

Only works for the keypad - action, the one for the keypad + doesn't work, very likely because I need to escape the + or something like that.