Simple question - can it be done?

Scenario: I have a process managing Sales Contract processing. At a stage in the process, I need to kick off some offline billings processes (bill, credit, cancellation) based on values captured in a form and stored in SmartObjects. These Billings processes can also be started manually, and so they have been built in a separate Solution File to the Contracts Processes.

When I add an IPC event, browsing for the child process only shows me the processes in the currently loaded solution file.

How do I use the process object to call and start a process built and deployed as part of a separate Solution file?

 Thanks

Paul