change UOM across UOM class

 Does anyone know whether there is a way to change the UOM to a different UOM which is defined under different UOM class?

Scenario -

A requisition was created in the iprocurement module, the Unit of issue of the line is 'Each'  The UOM class of 'Each' is 'Quantity'. 

At PO autocreate time, buyer wants to change the UOM from 'each' to 'foot', 'foot' is in 'Length' UOM class. 'Foot' does not show from the LOV. 

Any tips help.

Thank you.

Poching Shen