You can certainly add other registries to your wedding website!
First, you will want to head over to your Manage Website page here. Then, you will want to visit the Registry tab. Once there, just click on the button that says 'Add Existing Registry'. Follow the prompts to enter the name of the store where you're registered, then add the link to your public registry web address (the one you would share with your guests).
If you would like, you can add a note to your guests. After you're done, just click on 'Save' and it will appear as a link on your website.