When I add the host to the cluster do I need to put it in maintenance mode?
No, but you may need to shutdown running VMs, if the EVC level is lower than the host's CPU capabilities.
Also I would like to enable HA. I would need to make sure all my vms are in a shared storage correct?
Yes, only VMs which are which are accessibly from a surviving host can be restarted by HA.
Can I add a host with vms in local storage and later migrate them to a share storage?
Yes, no problem with having VMs on local storage.
André