In Service Studio you can use processes that are exposed by another eSpace, called, in this context, a Producer. Simply add these processes as eSpace references of your eSpace, called, in this context, a Consumer eSpace.
How to Add a Process Reference:
Once processes are added as references, you can use them in your eSpace:
In any process flow, by dragging the Execute Process tool from the Process Flow Toolbox, onto the flow, and choosing the referenced process in the Process property. A quicker way to do this is simply dragging the referenced process from the eSpace Tree into the flow.
In any actio flow, by dragging the Launch<Referenced Process Name> action from the Processes folder (on the Processes Layer of the eSpace Tree), into the flow, just like any other action.
Process references are executed in a manner that is most similar to the way action references are executed. See how.
Add/Remove eSpace References | Expose a Process | About Platform Server Session