Is it possible to set a custom build/firmware on a 2762n so i can configure the factory default username and password which stays when factory reset is done on the router?

Only if you place a custom order with DrayTek for new products - you can't do it yourself. I don't know how many you need to order, but they probably won't do it for one-offs because it ends up confusing people who can't reset an acquired device and then send it back for 'repair'