You cannot use DSCC to perform this task. Use the command line, as described in this procedure.
The following example shows how you can use the same add LDIF syntax to add values to existing multi-valued attribute and to attributes that do not yet exist:
$ ldapmodify -h host1 -p 1389 -D cn=admin,cn=Administrators,cn=config -w - Enter bind password: dn: uid=bjensen,ou=People,dc=example,dc=com changetype: modify add: cn cn: Babs Jensen - add: mobile mobile: (408) 555-7844
This operation might fail and the server will return an error if any of the following are true:
The given value already exists for an attribute.
The value does not follow the syntax defined for the attribute.
The attribute type is not required or allowed by the entry’s object classes.
The attribute type is not multi-valued and a value already exists for it.