I'm not aware of a way to import/integrate existent snapshots from Nimble arrays to act as the source of Backups/Archives data for the Veeam configured jobs.
However, considering the existent storage snapshots aren't the object that Veeam Software is aware of, the "import" function is unlikely to be preferred as to the most, the existent snapshots and structure of volume groups in place without Veeam in the picture (yet) would differentiate from the structure of jobs/copies/snapshots to be taken once Veeam is plugged into directly integrated snapshot capabilities.
Product Architect @ SingleHop - Veeam Platinum Service Provider
VCAP-DCD, VCAP-DCA, VCP-NV
Veeam Certified Architect