set attribute system alias user/password

Hi

in a small java application i tried to set the system alias access.

I have my IUserMaint object. With this object i can get/set moste UME Attributes like name, adress, company etc.

Now i want to set an access to a system_alias and is try to set the username and password for this System.

I tried following code snippet in different ways, but without success

user.setAttribute("com.sap.security.core.usermanagement", "$usermapping$:SAPERPKG:user", valueArray);

or

user.setAttribute("com.sap.security.core.usermanagement", "SAPERPKG:user", valueArray);

I want that my java application can set username / password for a system alias in /useradmin under the tab "user mapping for system access", how can i do this?

Thanks for help