jQuery Grid Plugin - jqGrid

There are items with hierarchical categories. It's about 15000 items. What I want to accomplish is to show categories at first as a tree. Then a user can expand categories until he/she gets items.
Multi grouping looks pretty good. But as I understand groups are come up from fields that are loaded in the table only. So on the first page where there are 500 items jqGrid doesn't show the full list of possible groups/categories. It doesn't know about all possible groups in the actual DB table. My question is is there a way to force jqGrid to show all possible categories at first and show all possible categories? The second question is is there a way to load items dynamically? I want to load items only when user expands all categories (by demand). I know that jqTree can load items dynamically, but is it possible in jqGrid? Thank you!