Help me clear in design

Hi experts,

I'm new member in LDAP.
Although I've finished configured a Linux server qmail - ldap but I still have some points unclear, let's I explain:

For example, I have a lap using some services require authenticate, ex: qmail, radius, ftp, ...
I want to save the user information (username, password, ... ) in only one place, so I decided to using LDAP. (number of users are quite large)

But after implement qmail-ldap (it now runs good), I don't know how to continue to implement radius service (or other services). Reason is I can't integrate the radius information into qmail database (although I included qmail.schema and radius.schema in slapd.conf).
The question: is there a way to integrate data of one schema in to an existed schema?

I want to design a structure like this:

    [qmail information]           [radius information]             [other service ....]
                  \                                     |                                       /
                   \                                    |                                     /
                     \                                  |                                   /
                        \                               |                                 /
                                          [common user information]
                                          (username, password)

Do I have to write my own schema for doing this? (Schema that include both [qmail service information] and [radius service information])

Please give me an advice how to solve this?
Thanks,

--
Phan Dang Duy Thinh