Yes, you can use an existing domain with your G Suite order.
When switching to G Suite from another program or web service, you and your users can bring your existing mail, contacts, and calendar data with you. You have a variety of options for migrating data into G Suite, depending on the size of your organization and the system you're migrating from. Tools are available for migration from Microsoft Exchange, Lotus Notes, IMAP servers and other Google accounts.
Each email account comes with 30 GB space.
In addition to accessing G Suite mail from the Gmail web interface, you can send and receive mail from your favorite desktop client. Depending on the client, you can use either the IMAP or POP mail protocol. If youâ€™re switching to G Suite from Microsoft Exchange or some other Outlook service, you can use G Suite Sync. This is a plug-in for Outlook 2003, 2007, 2010 or 2013 that lets you use Outlook to manage your G Suite mail, calendar and contactsâ€”along with your Outlook notes, tasks and journal entries.