Thanks !
I used the webservices solution, and it works great ! I use the Portal_UserService webservice, already present in Liferay, and it does what i want.
The reindexation is automatically done, and after the creation / updates of users, i can directly log in with CAS server, and the user is logged in in Liferay.

But I founded a problem and I don't know why it is like that :
We use CAS Server so we have a CAS Filter in the web.xml that redirect guest users to CAS server login page automatically
(we don't want any guest access on any page of our Liferay portal)
BUT :
In guest, i can access the page :
http://127.0.0.1:8093/tunnel-web/axis
that list every webservices ! In this page guest can see WSDL ! Why ? and how can i change that ?
it's strange because if i type any kind of url, i'm redirected to CAS Server, but not this particular page.
Firmi prego dentro per inbandierare questo come inadeguato.