If you feel comfortable writing some Apex code, you could write a small class that creates the approval process, which you could call from the Flow using the Apex callout element.
E.g.
Apex Class: 'SubmitForApprovalPlugin' implementing the Process.Plugin interface
Flow calls 'SubmitForApprovalPlugin' using an Apex callout.
If you feel comfortable writing some Apex code, you could write a small class that creates the approval process, which you could call from the Flow using the Apex callout element.
E.g.
Apex Class: 'SubmitForApprovalPlugin' implementing the Process.Plugin interface
Flow calls 'SubmitForApprovalPlugin' using an Apex callout.
inputs to apex class (as a value table):
* Id (of the record to submit for approval)
* Comments (comments for the approval request)
outputs to Flow (as a value table):
* Success status (boolean)
References:
Developing an Approval Process in Apex: http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_process_example.htm
Implementing the Process.Plugin Interface: http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_process_plugin.htm
Designer user guide (there's a section for the Apex Callout element): http://www.salesforce.com/us/developer/docs/firefly/salesforce_flow_designer.pdf
Let me know if you have any questions, hope that helps!