Hello !
While there is nothing preventing you from expanding the cluster which was deployed through Intersight using the on-prem Installer, we have seen this cause future issues with some management operations through Intersight (mainly Upgrades).
The recommendation is to use the Intersight to expand the cluster that was deployed and managed through Intersight, to avoid such conflicts.
Kind regards,
Aleksandar
Cisco TAC - HyperFlex Team