@Override
    public void insertProperty(String username, String propName, String propValue) throws UserNotFoundException
    {
        // all providers are read-only
        if (isReadOnly()) {
            throw new UnsupportedOperationException();
        }
        getUserPropertyProvider(username).insertProperty(username, propName, propValue);
    }
Returns whether <em>all</em> backing providers are read-only. When read-only, properties can not be created,
deleted, or modified. If at least one provider is not read-only, this method returns false.

@return true when all backing providers are read-only, otherwise false.
xmppserver/src/main/java/org/jivesoftware/openfire/user/property/UserPropertyMultiProvider.java