If the field is just for display purposes you can use CBSubs Plans Image field which can output an image based off their subscription. You can also use a Custom HTML field with content prepare enabled under the Parameters tab then use the CBSubs substitution usage as follows.

[cbsubs:subscriptions plan="PLAN_ID_HERE" output="expiry_date" /]

Another alternative is using CB Query Field to query for the expiration date or CB Code Field to utilize PHP and CBSubs API, but those usages become a bit more complicated.

If you need this to be searchable you'll need to use a standard datetime field then set its value using CBSubs Fields or CBSubs SQL Actions on plan activate using [SUBSCRIPTION_EXPIRY_DATE].