What is a common attribute similar to objectGUID?

Hi,

We want to use a non-used attribute in a destination domain which can contain the objectGUID of a source domain based on a FIM Sync.  We want to use one which is already present in the Directory with not having to extend the schema.  The objectGUID is a OctetString defined attribute.  This is a 2008 R2 Forest/Domain.  Any suggestions on attribute options?

Thanks for your help! SdeDot