You may try Manage available packaging options per products. Go to Inventory -> Configuration -> Setting -> Products -> Packaging Methods.

After enable it, you may go to your product, and have your new UoS on Inventory -> Packaging. You may set the UoS coeff there. After filling the information, you may sell your products with Extended Sales order lines on packaging with your new qty.

I am using Odoo 9C F.Y.I. wish it helps! 

i think you have to set the Purchase UoM.

i've enabled the features through the user setting.(manage multiple unit of measure)

maybe there are other ways to enable it.