Hi Laura,
Your valueexpression formula is rendered "on the fly", so technically, you could add
sortOrder=1 sortType=desc
to the column (vs querysort, which I think of as What To Sort By When A Column Header Is Clicked). However, depending on Workfront's order of operations in rendering the grid, it might ignore the sort (eg sort first by nothing and then calculate the cell contents on the fly).
If so (returning to my previos suggestion), by instead moving the formula into a calculated parameter on a custom form, the calculation is "known", and when rendered in a grid (e.g. using valuefield vs valueexpression), the sort will then take effect.
Regards,
Doug