This is really disappointing.
The software we are moving from had a way to do it in comjunction with crystal reports. We had the report include the length, and an extra field that had a 1 in it. Whenever the 1 was encountered, it took the length value instead.
The result was a total BOM of wireways by length, counted separately in mm (our current unit of measure) by the different types of wireway used in the panel.
What we'd also done was create a VB program that took the data and entered it into our accounting software, so the manipulation of data into the accounting BOM was not too bad.