That's very easy:
Edit /opt/webadm/conf/objects.xml. You will find here all the objects
you can create with WebADM.
You can modify existing object definitions and even add new.
In your case, go to the XML block starting with : <ObjectClass
name="$webadm_account_oclasses"
And add your class "tt4Account" to the auxclasses : auxclasses="user,
person, inetorgperson, securityprincipal, tt4Account"
This is for the WebADM Accounts and you can do it for normal user
accounts by doing the same in the <ObjectClass name="user"
If you want some specific attributes of the class to be part of the
"Add Attribute" list (in the WebADM user edit) and proposed as
optional attribute (in the WebADM creation/extension forms), then you
can specify your attributes with an "Attribute" XML block in the
objects.xml.
Restart your WebADM and you will have the class added to the creation
forms.