Edit user selection of dialog

Hello, 

i'm trying to edit a user selection a user did in a dialog. This dialog is part of an executable process in the ARIS Process Governance. 

Unfortunately, i can't find a possibility to add users to this selection or to delete existing ones via localService or report. 

Can someone help me?

Many thanks in advance! 

Regards, 

Richard 

For discussion, in this example i just want to add someone from UMC to an existing selection:

var me = userManagement.findUserByLogin("loginName");
var myDisplayName = me.displayName;
var myId = me.UUID;
                    
function isUserChooserEmpty(selection) {
    try{
        selection.participantGroup.participants.participant[0].id.idValue;
        return false;
    }catch(e){
        return true;
    }  
}

if(!isUserChooserEmpty(input.userSelection)){
    var users = input.userSelection.participantGroup.participants.participant;
    users.add(me); //This is not working because the object is of type AInputXMLData. Here is the problem.
    output.updatedUserSelection = users;
}else{
    output.updatedUserSelection = "updatedUserSelection is empty";
}