If you are saving your form to SharePoint, SQL or a Web Service, just add a button to your form and configure it to Submit to your SharePoint site. It will require the form is valid to submit. To let them save the form even when required fields are missing, you can use qRules ClearErrors command on the button rules to override (but don't forget to get the errors back on open).
If you want something that saves to a local directory - that will require full trust. I don't recommend doing that. It's not worth the trouble. But, if you must, you just need to add a function that calls the Save method. Your button can then call this method via rules (trick to do this is set the call in the condition of the rule).