Additional software would be necessary to be able to perform this level of deduping.
Support can add a second de-dupe field. Two big things to know:
1. This adds a second field in addition to the standard email address. So your lead's email address must be an exact match, and this second field would also need to be an exact match as well. If either field is not an exact match, it will create a new lead record.
2. A NULL value counts as a value. So if you have a lead with an email address in the standard email address field and nothing in your second field, then have a new lead coming in with an email address and anything in the second field, the anything will not match the NULL value, so a new lead record will be created in that scenario as well. Same goes in reverse--if the incoming lead has no value in the second field but the existing lead does have something, they won't match and a new record will be created in that scenario as well.