Ok, thanks for your answer.
I´m using the Instant GUI config, så I have found out that this is not possible.
But I have found that the IAP by default send the cluster name in the request when I use Dynamic Radius Proxy settings.
I have also fund, by help from Aruba, that in IAP release 6.5.4 and AMP version 8.2.5 I have the possibillity to configure separate radius servers for authentiaction and for accounting that will solv my issues verry well as it seems, at least in teory. :-) to be tested . . .