H Lizzie,
To my knowledge, you can upload a CSV contact file from Outlook Web App to your own contacts list. However, the contacts list is not able to be shared in Outlook Web App. (If you have an Outlook client, then you can share the contact folder.)
In Outlook Web App, if you are the administrator in your organization, you can choose to import these contacts via Windows PowerShell into global address list which is a “centralized address book” in your organization. The steps are as below:
1. Create a CSV file that contains information about those contacts.
2. Create the external contacts with PowerShell.
3. Add information to the properties of each external contact with PowerShell.
For detailed steps and commands, you can refer to step1- step3 in this article:
Bulk-create external contacts in Exchange Online
If anything is unclear, please post back. I will do my best to help you.
Regards,
April Zhou