We usually try to assign users in Org Strctures either by creating a new postion and assigning the user id to it (or) Drag and drop.
There is one more way by which you can assing the user which is there in Org id A to Org id B. This is a standard procedure and you can follow it
Go to Tcode P013 and input the postion Number
Enter the postion number and select "Relationship and clcik on "Edit"
In the above screen shot you can see that there is a filed called "ID of the related Object". This is actually the link between the Org id and the Postion. So for example if it is assigned to Org id 500000 and if you input 500001 in the filed then the postion will get assigned to that particualr ID.
Instead of Drag and drop you can directly do it this way and the tables that store these relationship are as follows
Here also via bebugging you can change the Id of the related object. But by doing PO13 is a safest way and easiest.