Is there any way to have a Flex Volume inside a Persistent Volume Claim?

Any type of PV can back a PVC. You need to create the PV by hand and then specify the name in .spec.volumeName of the PVC (or use .spec.selector with labels). Like so:

kind: PersistentVolumeClaimapiVersion: v1metadata:  name: task-pv-claimspec:  volumeName: task-pv-volume  storageClassName: manual  accessModes:    - ReadWriteOnce  resources:    requests:      storage: 3Gi

As a reference I used this PV (but the type of the PV does not matter):

kind: PersistentVolumeapiVersion: v1metadata:  name: task-pv-volumespec:  storageClassName: manual  capacity:    storage: 10Gi  accessModes:    - ReadWriteOnce  hostPath:    path: "/tmp/data"

(Alternatively, automatic provisioning with your own storageclass is also possible, but I guess this is not your use case.)

Yes.

apiVersion: v1kind: PersistentVolumemetadata:  name: pv0001 spec:  capacity:    storage: 1Gi   accessModes:    - ReadWriteOnce  flexVolume:    driver: openshift.com/foo     fsType: "ext4"     secretRef: foo-secret     readOnly: true     options:       fooServer: 192.168.0.1:1234      fooVolumeName: bar

Ref: https://docs.openshift.org/latest/install_config/persistent_storage/persistent_storage_flex_volume.html