Why is it not possible to create an Exchange Online mail enabled distribution group in Identity Manager? (4271011)

An Azure AD group cannot be converted into a mail enabled distribution group.  When creating an Exchange Online distribution group, the top field "Azure Active Directory Group" is always greyed out. 

There are two different PowerShell commands to create a Distribution list:

1. Enable-DistributionGroup

Use the Enable-DistributionGroup cmdlet to mail-enable EXISTING universal security groups and universal distribution groups that aren't already mail-enabled.

This cmdlet is available only in on-premises Exchange.

2. New-DistributionGroup

Use the New-DistributionGroup cmdlet to CREATE distribution groups and mail-enabled security groups.

This cmdlet is available in on-premises Exchange and in the cloud-based service.

That means for the cloud, Microsoft provides only the second option, and that creates both objects.  Therefore it is not currently possibility to create a Distribution List for an existing group in the cloud.  This is also why this is not possible in Identity Manager.