It's very interesting business case. Aria Sourcing(AS) have not capability to communicate DocuSign itself. but I try to describe 3 workaround options below;
#1. DocuSign standalone
AS gives award mail described send back address of Buyer side signer to the supplier normally. also supplier send back request for Award certificate form that described by supplier own to signer.
Signer gives eSignature on Award form and send to the supplier. Signed form can be store on MM or buyer's file storage. it's very easy solution but can't avoid manual processes with NIGO.
#2. MM manages eSign workflow
When AS acknowledge awarding supplier and condition to MM, MM generate Award certification form communicate with DocuSign and send to signer. Also when signer eSigned off, automatically send CC to Supplier and store to MM or buyer's file storage. It might be most elegant process but need to system integration.
#3. Expand award process to Ariba Contract Management(CM)
When awarding over, buyer create contract workspace inherited contract terms from AS. CM can have template include eSiguneture capabilities and Award certification document. You must be send eSigned form to Supplier using standard functionality of Ariba. Also CM can communicate MM, eSigned file will be store to MM.