Answer
Only within specific limits — Custom Actions support calling methods that take simple scalar parameters (VARCHAR2, NUMBER, DATE) or no parameters at all. They don't support methods that return data (functions, or procedures with OUT parameters), complex PL/SQL record-type parameters, BLOB/CLOB parameters, or PL/SQL methods that are exposed through a projection's _SVC package, a client-metadata _CPI package, or a report's _RPI package. If what you need falls into one of those unsupported categories, a Custom Action isn't the right mechanism — you'd need a different approach (like a proper projection action) instead.