Hi JCD,
Each specification has its own identifier category and type (one unique name or number like CAS & UN), why do you want to add the identifier listing of one specification to another specification (means new one) ??
Identifiers are related to spec. cat not spec. type. Spec. type is is which form substance do you want to use real_sub or pure_sub etc.
If you want attach, you have to create a new specification with new specification type (if the identifier list is differs from existing specification id's ) and its based on spec, category only specification type.
Thanks
Satya