1) Check out the reprot Library on CustomerSource - MS has provided many additional reports for download.
2) Check back in this newsgroup, Report editing is discussed frequently.
3) Here's the simple instructions to add an Overstock column - you need to do the calculation in a column, not in a filter.
Open the ...\Store Operations\Reports folder make a backup copy of "Items - Item Quantity List.qrp" right click "Items - Item Quantity List.qrp" and select properties, then uncheck "Read - Only" Open the Item Quantity List in a text editor (Notepad)
go all the way to the bottom of the file and paste this block:
Begin Column FieldName = "OverStock" DrillDownFieldName = "" DrillDownReportName = "" Title = "Overstock" VBDataType = vbDouble Formula = "(Item.Quantity - Item.QuantityCommitted) - Item.RestockLevel" ColHidden = False ColNotDisplayable = False FilterDisabled = False ColWidth = 1215 GroupMethod = groupmethodNone ColFormat = "" End Column