You can add a Constituent Code mapping via the Constituent area. There are no required fields here but you have the option to map the code itself as well as date to/date from. In the example below the field settings have been configured to use the dynamic date options to select the current date.
Within the area settings of the Constituent Code mapping you can also determine when to add a constituent code where a match is found (eg. always add regardless of match, only add if no match or never add if a match is found). There are options to specify whether to add constituent codes to new/existing constituents as per the image below.