You can operate a DNS zone as a secondary with the master in a legacy DNS system.  You can also update DNS records from the DHCP service in a system other an Infoblox.  This might work during transition, but there would not be any value in operating all of your DNS and DHCP services on other systems or integrating Infoblox as only a secondary DNS.

Also, be aware that you cannot directly convert a secondary DNS zone to a primary.  You can create a new zone on Infoblox for an existing zone in legacy platform, enable zone transfer from the legacy platform, and Infoblox will create the zone, perform a zone tranfer, and populate the transferred records into the new zone.

If you are looking for great IPAM, but want to keep Microsoft DNS/DHCP, maybe a purchase of Infoblox IPAM for Microsoft is a better fit?

I hope this helps some!?!?