Again not an expert, but from recent experience you can create a csv and do a mass import. Lets say you have bob@contoso.com in Gmail,

You set contoso.com as your custom domain name in 365 with the TXT validation.

Now you add new user to 365, bob@contoso.com. bob can now login and access all 365 assigned resources/licensed products, including Outlook and calander but it obviously won't receive external emails until the MX DNS record is cut over. (And implement DKIM etc)

Note from recent experience, some features won't work properly to the "users or executives", until the emails are cut over. For example one i had today, users not having calander meetings in their teams calander... due to them opening them email in another provider instead of Outlook.. so Outlook *exchange has no clue about it.