The finished product location can be selected on the miscellaneous tab of the MO

Note this can only be set while the MO is in draft state

Your question seems to be specific to if this can be set after completion of MO. In that event an internal transfer might be required. 

If this is a frequent occurrence, you could consider to create a push route from WH/Post production to WH/Stock. While transferring product the products employee can specify the sub-location on the transfer