Hey Wolfgang,
you can set almost any UDM attribute via the command udm create --set as shown in the manual.
As stated in Q&A: Which mail fields for users are available in UCS?, the contact mail address corresponds to the UDM attribute e-mail, so the invocation would be:
udm users/user create --position cn=users,$(ucr get ldap/base) --set username=user --set lastname=Testuser --set password=test --set e-mail=user@example.com
Best regards