Hey JiHel,
Off the top of my head I think you can:
1. set the “hidegrid” option to false to remove the collapse button – you need to have a caption for this to work.
2. You set subgrid column headers the same as you would a normal grid.
3. Set “autowidth: true” in your subgrid definition
4. Check out the subgrid demo to see how to expand and collapse subgrids. http://trirand.com/jqgrid/jqgrid.html