How to cost-load a schedule using Excel export and import

I want to export a schedule to Excel, cost-load the activities there, and then import it back into the schedule. How is this accomplished?

I know how to load non-labor costs in a schedule manually. I use a non-labor resource "Lump Sum." I also know how to export the schedule as an xls. But I can't get the costs to import back into the schedule after I've added them in Excel.

Thanks,

Rob