can you describe us your BPM Process?
Referring to this sample:
Main Process (Default):
Sub Process (subDefault):
There are two types of Subprocesses: SubProcess and Embedded Subprocess. The first type refers to an external Process while the second one could be edited by the "standalone" editor.
For the Subprocess you can choose the Pool of the other Process by set the Pool Property in the Properties View
For Embedded type you can open the "standalone" editor by selecting Edit Standalone item from the context menu.