Hi – thanks for posting!
If your theme does not support standard nav menus, I’m afraid there’s not much else I can suggest – simply because I’ve no way of knowing how your theme is implementing its menus or what filters might be available.
Would it be viable to set up a custom menu widget, thus letting you take advantage of WordPress’s native nav menus even if your theme does not support them directly?
Beyond that, I’d ask if you have any further technical questions that you take them across to our wordpress.org support forums as we don’t normally deliver technical support here on the pre-sales forum.
Thanks so much!