FormsCentral doesn't support digital signature.
You can create the form you need in FormsCentral and then use Acrobat and EchoSign to collect, complete and sign the form (I'm not quite sure I understand your workflow though).
About EchoSign : https://echosign.acrobat.com
Acrobat Form Forum : http://forums.adobe.com/community/acrobat/form
If I send the form out and the customer submits it, how do I receive a copy of the form back? Currently, the data shows up online in a file. I'd like to be able to collect data online, but I will still need to approve and sign digitally to return to the customer.
If you have a paid account you can right-click on a row in the View Responses Tab and "Download Response as PDF Form..."
You can then save this PDF on your machine, sign it (for example using EchoSign or with a signature field in Acrobat) then manually email the signed PDF with data to your customer.