SELECTED ANSWER
replied on January 7, 2016
Unfortunately, no. Forms does not currently support setting variables on an arbitrary process instance. Workflow can only send data back to Forms on the workflow completion message. You may notice that the description of the activity says "Sets an output parameter with variable values that will be processed by Laserfiche Forms when this workflow completes." So this only works when the workflow is started by Forms and the service task activity in Forms is set to wait for the workflow completion.
What you can do, is set the first workflow to wait for the invoked one. The invoked workflow can pass output parameters to the parent workflow, which in turn, can send them back to Forms when it completes.