You don't need to use a contact field to do this. In the final stage of the contact list upload you have an option to create a new Shared List or add the contacts to an existing one.
I recommend that you name the upload accordingly and in the final stage select Create as New. This way you can always come back to the list or use it in segments or campaign steps.
Of course you can always create a contact field just for this info, add that field in the upload file as a separate column and populate the column the way you want. In the upload map the column to the contact field you just created. But I recommend not to use the contact fields for ad hoc purposes like this. Shared lists are much more useful and easier to manage.