How to "Register an older product" in Creative Cloud

Hi,

My organisation has a number of licenses and we are currently under an ETLA agreement.  In the LWS I can see the serial numbers for what we have purchased and I wanted to make them available to some staff as a trial via the Creative Cloud.  When I try to enter in any of the serial numbers, I get the message to "Please enter a valid serial number".  My organisation bought a number of Creative Cloud licenses and we want to make use of them.

How should I proceed to resolve this.

Cheers

David