VMware Cloud Foundation is a Greenfield installation only.
There are several options for migrating workloads from your legacy vSphere environment. Including NSX Hybrid Connect (HCX), vSphere Replication, Cross vCenter Workload Migration (Fling), and using NAS storage. A document on how to use all these migration tools will be coming soon.
If you have vCloud Suite Licensing those license keys can be upgraded to VMware Cloud Foundation, ask your sales team for help with that process.
Heath Johnson
VMware Technical Marketing - ISBU