The available checkboxes and menus are limited to a certain number for each kind of control in the displayed entry.

Generally, the only way to get more checkboxes is to create a second object which is bootstrapped by the main object, and add the extra controls on the second object.

An example in the pathfinder core data files would be the headband of vast intelligence +6, which bootstraps a second item with the third menu to select another skill.

Ok, I'll give this a shot.