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.

Area Settings

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.