Here is a hypothetical use case. i am wondering how this might best be accomplished?
A document gets distributed to 2 other people and everyone signs it. A bit later a new signature is required so that everyone knows that that additional person has "approved" the document.
Example - the original team signed off and now another manager has been brought into the project and needs to add his/her signature to provide an approval trail.
Is this currently possible?
I don't think that's currently possible. The way the system works currently is that all required parties need to sign the document before the deadline. If you upload a document that has already been processed by the system (e.g. all parties signed before the deadline and the PDF with sigs was generated), it won't be accepted since it is encrypted.
It sounds like you are looking for a workflow component with multi-level signatures. That isn't currently part of the eSignatures service, but Adobe does offer server products (LiveCycle, on which the eSignature service is based) that can do this. Maybe in the future.