You can certainly buy the keyboard tray separately at a later date. When you’re ready to make the purchase, reach out to the support team with your current shipping address, original invoice #, and the color you are interested in.
Once that information is received, you will be sent a custom invoice via email where you can complete payment. Make sure to reach back and let us know once you've done so and we can then add the order to the queue for processing.
Please note that tray purchases can take anywhere from 1-3 weeks to ship depending on demand.