Admin category tree : has it been done ?

Hello,
I'm currently working on a migration from OsCommerce > OpenCart for a website with more than 500 categories.
As you can imagine, the regular navigation is not very useful there.
Is there an extension that creates a category tree in /admin/categories (and/or in /admin/products) ?
If not, has anybody worked on something similar before ?

Thanks for your advice.

Maybe the free import / export tool might help or make it easier?

One warning before you try..... The free import/export tool erases all the categories and products then imports what is in the spreadsheet so make sure you make a back-up of all your work before you attempt an import.

Mike

Ok, I might have been unclear.

The 500+ categories are all imported and running ; on the front side , everything is ok (well, once you deactivate the product count).
But on the admin side, admin/categories is not usable. Too many pages.
I'd like to have a tree, like the tree in image manager (on the left, click on the directories, on the right, images appear).