Mac Clone - how to add specific mac addresses?

Hi, i´m trying to configure a new ar750s for a client. Said client wants to use the MAC Clone feature to change to specific mac addresses other than the factory one, the current device´s one or a random one via the dropdown list. The reason for the easy accessibility is that my client has very limited computer skills, spoofing manually via the terminal would be impossible to do for him. Is there a way to add extra MAC Adresses to the dropdown list to choose from?

The two ways cannot meet your client need?

If said client is unsophisticated - why do they want to spoof MAC addr in the first place?