After migration you will need to manually add shared mailboxes. Use the following steps to add your shared mailbox:
For new Outlook:
First select Mail from the navigation column on the left side of the screen.
In the Folder pane right click on your account name and select Add shared folder or mailbox
In the Add shared folder or mailbox window, type the name of the mailbox you want to access.
You should now be able to access the shared email in the Shared with me folder. The mailbox will appear as a subfolder under Shared with me. When you select the name of the shared mailbox it will expand to show the standard email folders.
To add a shared mailbox on Classic Outlook:
Open classic Outlook.
Select the File tab on the ribbon, then select Account Settings > Account Settings.
Select the Email tab.
Make sure the correct account is highlighted, then choose Change.
Choose More Settings > Advanced > Add.
Type the shared email address.
Choose OK > OK.
Choose Next > Finish > Close.
The shared mailbox should now be added and visible on the left pane with all your email folders. You may have to restart Outlook for these changes to take effect.