Prior to v4.0, only third party or BP can provides a tool to migrate.
1.
JazzConnect – HPQC Edition developed by CM-Logic, is an application that
provides plug-in based support for the automation of work teams between
Jazz-based applications such as Rational Team Concert, Rational Quality Manager
and HP Quality Center.
See more info at:
http://www-304.ibm.com/partnerworld/gsd/solutiondetails.do?solution=46271&lc=en&
stateCd=P&page=&expand=true
2.
It can be done with help of Sogeti.
Sogeti provides a service to help you migrating from HP-QC to RQM.
See more info at: http://business.vanlint5.nl/sogeti/

From v4.0, there is IBM internal tool which can be used by the service team:

See this technote: http://www-01.ibm.com/support/docview.wss?uid=swg21507194

from which, it also has an article in developerWorks explaining how to use RQM Excel Importer add-on
to do the migration job(from HPQC to RQM only though)