Joomla 1.6+ menu items are stored under the #__menu table, with a special menu type called "main" for the admin menu.
Find your main component admin menu item's ID. You can add sub-items of that by declaring the parent_id column as the id of your main menu item, and setting the level column to 2.
The only other issue you are going to run into is the adoption of nested sets (lft and rgt columns). This is better way to handle parent-child relationships and the ordering of menu items. I'm unsure whether or not the parent_id or lft/rgt are used at this stage, but they are both filled in.
To add a new item, you would have to "shunt" all lft/rgt values by two for menu items with a value greater or equal to the position you wish to add your menu item. This should include the rgt of your parent item. If your parent item has no children, the lft for your new item will be the value of parent's left + 1. The value of the new item's rgt will be parent's lft + 2.
One thing to note with the lft and rgt is that the numbering applies to every menu item (front-end and back-end), so not doing it properly may have the potential to break your entire menu heirarchy. I think this is why the parent_id column is still used, and that there is an option in the admin area to "rebuild" the menu.
http://en.wikipedia.org/wiki/Nested_set_model