Vegard,
As you say, the obvious way to solve this problem is through alternate sales Unit/M. It would seem to me that a warehouse should have no problem with picking/packing/shipping the same material using two different U/M; this is one of the main functions of warehousing. However, you seem to think that your warehouse people can't handle this.
Since that is off the table, then I would say that the most common way to turn one material into another material is through a production order. You put your ratios into the BOM. 1 B = .1 A.
Best Regards,
DB49