AS9100 is a prerequisit to Nadcap certifications for special processes, but there is an alternative AC7004. AC7004 is based on AS9003 but is phrased as a question checklist rather than "you shall" requirements typical of qms standards. AC7004 can only be audited by PRI whereas you can choose from multiple registrars for AS9100. I recommend AS9100 because it is more reputable and is more current with process-based methodology, risk management, customer service, and many other best practices not in AC7004.
If you are seeking Nadcap certification to any special process, you'd need a dated valid AS9100 certificate, or you could ask PRI to conduct their AC7004 audit even the day before the rest of the Nadcap audit.
Materials Testing Laboratories and Non-Destructive Testing Laboratories that are Independent Labs (not captive labs) need ISO 17025 as the prerequisit qms.
If you have other questions, let me know. David Johnson - see my profile for contact information.