can I change this?

Because of how EA made the design tool work it is not possible to group items on the same catalog entry without losing design tool functionality in the game. This issue has been brought up to the EA representatives at the official EA forums but they've shown little interest in working on changing it and doing so would, most likely, require a fairly significant rethinking of object .package structure. At this point grouping things onto a single catalog entry is, thus, a trade off. You get a more rationally organized catalog but it comes at the loss of design tool functionality for any of the regrouped items.