Migrationwiz tool is used to migrate mailboxes from GroupWise to Office 365
Lateron, they' would like to do some administrative tasks like adding a proxy address or putting a mailbox on litigation hold etc. Also they needed to edit most of the mailbox settings and they need their AD schema to be extended and they don't want to use ADSI EDIT to do these tasks.
Do they require Exchange 2016 on-prem server to do some of administrative tasks in future?
If yes, do they require Hybrid setup so that they can get license key of on-prem server through Hybrid setup?
Hybrid is not a requirement here. Exchange Server is not a *technical* requirement only, but if you want to stay in a *supported* configuration, the only way to manage Exchange related tools and attributes is via the Exchange tools. If you are fine with not being in a supported configuration, you only need to extend the schema then use whichever tool fits your needs.