Hi,
"I know that it is possible to assign storage locations of different plants to one warehouse number in SAP EWM. But I have some doubts if this is an adequate design."
There should be no problem doing that.
"Would it be more adequate to create a separate plant for depicting the central warehouse?"
That would probably mean more complex (manageable, but definitely more complex) processes for all movement, as you would always need stock transfer orders.
Brgds
Juergen