What is the reason that you believe you cannot? As far as ASME Sec IX goes, the ONLY organization that can certify a welder is the Manufacturer. Other parties may prepare the coupons or specimens etc.
As I indicated above, what product do you manufacturer, build or repair?
Your asking a loaded question for which the answer is two parts.
1. By the code alone, yes you can.
2. Some quotes from section IX you need to be aware of:
"Participation by federal agency representative(s) or person(s) affiliated with industry is not to be interpreted as government
or industry endorsement of this code or standard."
"This code does not address other safety issues relating
to the construction of boilers, pressure vessels, transport
tanks and nuclear components, and the inservice inspection
of nuclear components and transport tanks. The user of
the Code should refer to other pertinent codes, standards,
laws, regulations, or other relevant documents"
Pay particular attention to the usage and context of the words Government, laws, and regulations.
While Section IX usually is sufficient in the states, it is not always sufficient in a stand alone capicity for other countries.
There may in fact be a hidden requirement in the laws of your home country that require a third party, or in relevant standards,
and regulations.
Regards,
Gerald