I gather from your question that your laboratory is already accredited? Adding any test method to a scope of accreditation involves ensuring that all the ISO 17025 resource requirements (clauses 6.1 to 6.6) and process (clauses 7.1 to 7.11) are met for the particular test.
The laboratory should perform a full risk and opportunities analysis related to introducing any new test. For ELISA Assay Kits, you need to ensure that the most suitable technique (e.g. Sandwich versus Competitive ELISA) and applicable kit is used for the type of aflatoxins of interest. The laboratory must ensure that the method performance required for the purpose of the test is met. For example the test will only provide a total aflatoxin result for the aflatoxin antibodies used in a competitive ELISA test.
It crucial to use a suitable kit produced by a reputable company, with published validation an performance criteria. Then the laboratory will need, as with all tests, to validate the method inhouse before use. Remember that depending on the matrices, interferences must be addressed as ELISA test methods are not suitable for certain matrices, for example strong colour and flavours in concentrated additives.
The ISO 17025 toolkit at https://advisera.com/17025academy/iso-17025-documentation-toolkit/ can assist further. It includes the procedure for validation and verification of methods, named Test and Calibration Method Procedure, along a Test Method Development, Verification and Validation Register and Test Method Development, Verification and Validation Record. The techniques for method validation are listed as well as the required records.