Searching few domains for one uid

Hello, I've got 3 domains (departments), for egzample:

a.com b.com.de c.com.fr

and each of them has it's own posixAccount users (moved from nis). I want every user in each department to be albe to login with his credencials, localized under people.$(domainname) to machines under all domains, but as far as I know, You cant search for uid or anyting else in more then one domain, cause a BASE attrib in ldap.conf accept only one search base. Is the only way to do this is change the schema to: a.com.corp b.com.de.corp c.com.fr.corp and set the search base to dc=corp ? I've also read about setting base to config and put it there.