VERY easy....
1. There is a STANDARD task delivered by SAP to perform scenario form steps in the background. USE IT!
2. You could do this either (a) using the task to do a background form step to do this, but then your step would need a backend generic service config'd for it to do the "attachment". or (b) the easier way, just have a task execute your own custom ABAP method to do the attachment (it could read the process number that you bind to it, then pick up what ever...or all...form fields you need to then figure how/what to attach. To do the attachment, use SAP's own standard functions/methods for adding an attachment to a process(hunt down their code....don't have access at the moment to find it for you...pretty easy though).
Good luck!