I would like to share the solution.
Ive added the jsonRpcService to the main xPage:
sessionScope.put("stLoginPW", stPW)]]>
and add the call into the DojoLogin Custum Control
....
if ( dojo.cookie('DomAuthSessId') != null || dojo.cookie('LtpaToken') != null ) {
// Set Login parameter für Sametime
var deferred = setLoginArgs.set(dojo.byId('#{id:username}').value,dojo.byId('#{id:password}').value)
deferred.addCallback(function(result) {})
// we are logged in, but do we have access to the next url?
...
That's it
Thomas