Here is some relevant legislation pertaining to "conversion units":
Firearms Registration Certificates Regulations:
http://laws-lois.justice.gc.ca/eng/regulations/SOR-98-201/index.html
Relevant part "Conditions":
Section 4(2) Subject to section 5, the Registrar shall attach to a registration certificate that is issued in respect of a firearm the condition that when the type, action, calibre or gauge of the firearm is modified, the holder of the certificate shall advise the Registrar of the modification,
- (a) if the modification is intended to be permanent, within 30 days after the modification; and
- (b) if the modification is not intended to be permanent but still exists 30 days after it is made, without delay after that period.
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So, for discussion, since the modification of your firearm [9mm pistol] is not meant to be "permanent" [by adding & occasionally using a .22 conversion unit with it] then there is no requirment to advise the "Registrar" of the "modification" [addition & occasional use of the .22 unit with it].
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NAA.