This is same issue as https://issues.jboss.org/browse/JBPM-7063

1. Add Process variable, for example processVar
2. Add Subprocess to a Process (for example Event Subprocess), for example subprocessVar
3. Add any node inside of subprocess (for example Signal Start Event)
4. Try to add variable to this event using DataIO Editor

Actual Result:
Only processVar will be listed in Target listBox of Output Assignment
Expected Result:
Subprocesses variables will be listed as well as Process level variables.