After the initialization call to IASMEXT::Initialize, IISADMIN will call the IADMEXT::EnumDcomCLSIDs method to collect your DCOM CLSIDs. It will be called with index (dwEnumIndex) starting at zero and incrementing with each call until you return HRESULT_FROM_WIN32(ERROR_NO_MORE_ITEMS).
Your returned CLSIDs will be added to the list of CLSIDs from other IIS custom extensions in the MULTISZ metabase property MD_IISADMIN_EXTENSIONS (user type IIS_MD_UT_SERVER) under the path /LM/IISADMIN/EXTENSIONS/DCOMCLSIDS.
virtual HRESULT STDMETHODCALLTYPE EnumDcomCLSIDs(
/* [size_is][out] */ CLSID *pclsidDcom,
/* [in] */ DWORD dwEnumIndex
) = 0;