Hello! This has been in some ACI literature from early on and used as a selling point for gradually getting into ACI.
Essentially, You would stand up your ACI POD, with a seed leaf and seed spine (or more) and gradually move your connectivity over, while converting your standalone switches to leafs/spines. Depending on the type of migration some scripts on github.com/datacenter can help move the NXOS configuration to ACI.
I believe that there is a white paper in the works to address exactly this scenario that will be published to cisco.com sometime this year as well.
