Once players are in the CARDS rating scheme they are identified by one "bridge name" in all the clubs they participate. This is used in preference to a "number" like the ABF number as it only means another number to remember.
Even so the CARDS system will use the national ABF number to identify players if no other information is available about the player. Therefore there is no need to worry about identifying players who have an ABF number and play at other clubs where they are already participating in the CARDS system.
On the other hand there is an issue where you have players who do NOT have an ABF number AND are already participating in the CARDS system. In that case you will need to find out what their "bridge name" is from the other club or from CARDS at firstname.lastname@example.org. Once you have that "bridge name" go to that member's details in the masterfile and enter their "bridge name" before the next step.