From what I've found, you can't fill the contents of a dropdown list at runtime if the grid cell column's style is set at design time, crashing AutoCAD has been the result from this for me. By leaving it set to "decide at runtime", you can set the cell's style to dropdown list (or imagelist) and then fill it, all at run time. I.e.
(dcl_Grid_Clear Project1_Form1_Grid1)
(setq dwglist (list "drawing1.dwg" "drawing2.dwg" "drawing3.dwg"))
(setq nRow 0)
(foreach dl dwglist
(dcl_Grid_AddRow Project1_Form1_Grid1 (list dl ""))
(dcl_Grid_SetItemStyle Project1_Form1_Grid1 nRow 1 18)
(dcl_Grid_SetItemDropList Project1_Form1_Grid1 nRow 1 (list "Item1" Item2" "Item3")
(dcl_Grid_SetItemText Project1_Form1_Grid1 nRow 1 "Item1")
(setq nRow (+ nRow 1))
)
I realize my foreach example is setting the same droplist. You can easily change the list to something of an association list or maybe find out what the list should be during each loop. Either way, I am simply trying to demonstrate the set-then-fill-it idea.
Hope that's what you're after.