Add an objectClass to all entry

[Domenico Maimone <domenico.maimone@flexmail.it>]

Ciao All
I have following set up (in production):

     dc=example,dc=com
     |
     uid=pippo (with some objectClass)
     |
     uid=pluto ( with the same objectClass)
     |
     .........( with the same objectClass)


How to add an objectClass (present in the schema) to ALL entry, at the 
same time?

Can you help me?
Thanks
dmaimone