If you want just to be able to edit links, link names. You would just need to create hierarchical menu structure on database, add it's administration and change a bits of PHP code.
It would be a lot of work, if you want to have multiple menu groups, don't want it be bound to static position and you would be starting almost a scratch. How hard it would be depends on how system is build.
I might try to explain some main points if you decide to go with harder variant. As for easier one, it shouldn't be too hard to think of "how-to-do".