HI Amy ,
Basically in SRM we cannot achieve the central purchase orgainzation like MM. But, you can follow a kind of workaround as below.
Say, You have two purchase orgainzations 1 and 2 under company A.
Similarly, Another company B has purchase organization 3.Now, you can make purchase orgainzation 3 responsible for purchase organization 1 and 2 as well.
How to achieve that:
Create a purchasing group under Purchase Organization 3 and assign the org units for purchase orgainzation 1 and 2 as well in the responsibilities tab. In this way, the buyers of the purchasing group can do buying for Purchasing org 1 and 2 as well.