Add new entry to Java resource bundle from code

Im using Java resource bundles to localize my application and I have created a function which returns a message localized according to a resource bundle from a code... something like this:

public String getDescription(String code, ResourceBundle resBundle){
    String returnValue = null;  
    try{
        returnValue=resBundle.getString(code);
    }catch(Exception ex){
        returnValue = null;
    }
}

But, what I want to know if it is possible to add an entry to the resource bundle in case the code passed doesn't exist there, something like:

if(!resBundle.containsKey(code)){
    //This next line is pseudo-code... it is not valid at all
    resBundle.addEntry(code, "xyz");
}

Any help?