Hi John
Do you need to transfer the demand for all the plants at the same time? If so, I can tell you that this option does not exist on the standard system, but you can try to do that using a BAdI. Maybe BAdI MD_ADD_ELEMENTS can do the trick.
Otherwise, you can try to use a quota arrangement to do that. Using a quota arrangement, you can also define the special procurement type "stock transfer" and select different supplying plants, defining a percentage for each one.
BR
Caetano