Hi NoelBateman,
Do you mean that you have migrated all the other mailboxes, now only two room mailboxes haven’t been migrated?
If yes, we recommend you just create two new room mailboxes in Office 365 directly. And then, if the old room mailboxes have some important items that you want to migrate to the new room mailboxes in Office 365, you can just import them into the new room mailboxes. Here are the detailed steps and references:
1. Create new room mailboxes in Office 365: https://technet.microsoft.com/en-us/library/jj215781(v=exchg.150).aspx
2. Export the items from the old room mailboxes with the PST file: https://support.office.com/en-us/article/Export-or-back-up-messages-calendar-tasks-and-contacts-d791fe88-5c67-44c5-a3f6-a484508082a1?ui=en-US&rs=en-US&ad=US
3. You can grant the room mailboxes’ Full Access permission to one user mailbox, and then add the user mailbox in Outlook client. After that, the room mailbox also will exist in Outlook client. You can import the PST files into the new room mailboxes.
Set up email in Outlook 2013 or 2010 for Office 365: https://support.office.com/en-ie/article/Set-up-email-in-Outlook-2013-or-2010-for-Office-365-6e27792a-9267-4aa4-8bb6-c84ef146101b?CorrelationId=7caaec00-859c-41aa-9ccd-8d267fce09fd&ui=en-US&rs=en-IE&ad=IE
Import Outlook items from an Outlook Data File (.pst): https://support.office.com/en-us/article/Import-Outlook-items-from-an-Outlook-Data-File-pst-345ad0da-7bc0-4a02-9cb7-2d1cf48ba8e3?ui=en-US&rs=en-US&ad=US
Otherwise, could you provide more detailed information for assistance?
Best Regards,
Anna