If I understand your question correctly you would like to take multiple FormsCentral forms saved as PDF with the Submit button to submit the data to FormsCentral, and combine them into a single PDF retaining the individual forms submit functionalities.
If I understood correctly you could certainly create a PDF Portfolio from multiple FormsCentral PDF forms and each would work independantly. You can't likely create a single PDF with multiple page that are from multiple forms, Acrobat will combine all fields that have the same name which would break the submit on your forms. The PDF Portfolio would be the way to go.
Here is some info on PDF Portfolios: http://www.adobe.com/products/acrobat/combine-pdf-files-portfolio.html