Can tokens be self-referential?

For bookeeping, we have implemented a custom field stamping the date in which a lead enters a certain stage of the lifecycle (e.g. lead changed to MQL --> change data value MQL Date to {{system.date}} ).

In the case of some leads that recycle and go through the funnel more than a single instance, is it possible to append the current date with the original date to the same field? (e.g. lead changed to MQL, MQL Date is not empty --> change data value MQL Date to {{system.date}}, {{lead.MQL Date}} )?

Basically we would like to know each date that a lead enters a specific stage in the funnel.