I am unable to find a way, in Identity Manager's Management Console UI to add and objectclass to an attribute.
How do I add an objectclass?
There is no GUI option for modifying object classes. You need to export, edit, and import the directory.xml file.
Export the directory,xml file, modify it by adding inetorgperson to the user objectclass section, then import the directory.xml back in:
In the Identity Manager Management Console go to...
Directories > [your_directory]
Scroll to the bottom and click 'Export.'
Save the result directory.xml file and modify 'User' the section, like this for example,
<ImsManagedObject name="User" description="My Users" objectclass="inetOrgPerson" pagesize="0" maxrows="0" objecttype="USER">
Save the file and back in the management console...
Directories > [your_directory]
Scroll to the bottom and click 'Update'