Do CG active duty members who want to also become Auxiliarists have to complete fingerprint cards?

All Active Duty  personnel undergo a PSI of one form or another, at a minimum, including fingerprint checks.  That being the case, this information should be on record with SECCEN.

The Active Duty  person who is interested in becoming an Auxiliarist should work with their command and/or SECCEN to obtain their security clearance source documentation to provide as part of their enrollment process (and presumably then avoid having to undergo the Aux PSI process).