First, you may certainly use API 510 for pressure vessel alteration or the National Board Inspection Code (NB-23).
If you read the definition of an Inspector in the Glossary of API 510 it states
inspector: A shortened title for an authorized pressure vessel inspector.
authorized pressure vessel inspector: An employee of an authorized inspection agency who is qualified and certified to perform inspections under this inspection code. A non-destructive (NDE) examiner is not required to be an authorized pressure vessel inspector. Whenever the term inspector is used in API 510, it refers to an authorized pressure vessel inspector.