I think there is some keyboard interaction, but it is only to dismiss the seating dialog. So essentially you're right, the card is scanned and that causes the player to be bought-in, automatically seated, and a receipt printed.
Directly to your point, the scan should be able to invoke the "Add Players" dialog, select the player, and confirm the add + buy-in. I personally haven't done this, so I don't know what kind of "tweaks" need to be done in order to make this happen. It may be that there needs to be a "pause" of some sort between the hotkey that invokes the Add Players dialog (% in your case) and the player ID or name or whatever is being used to search for the player. The keystrokes most likely are being delivered to whatever window has focus, and if they are delivered too quickly after the %, then they end up being delivered to the Settings window, which ignores them. They need to be delivered to the Add Players dialog, which may take a moment to open.
The only way to close the Add Players dialog with a keypress is to press the ESC key, or with an Enter or Spacebar keypress while the Cancel button is in focus. Or, Enter or Spacebar is pressed while the OK button has focus, which it typically does upon open UNLESS the "Find" feature is enabled. If the Find feature is enabled, then the search field has focus, and Enter or Spacebar won't close the dialog unless the Tab key is pressed enough times to move focus to either the OK or Cancel button.
So maybe re-examine what is being sent via the scanner/bar code reader, and maybe add a pause after the % key is sent.