Hi Alison,
You can set up a campaign to append data to the text area.
For this example, lets call your current field "Aggregate entries"
First, create a new custom field (call it "New Entry") that is also a text area. Put this new field on your form. Then set up a campaign. In the Smart List, have the trigger "Data Value Changed: Attribute is: New Entry". In the flow, have the following step:
Change Data Value: Attribute is "Aggregate Entries"
New value: {{lead.Aggregate Entires}}, {{lead.New Entry}}.
This will take everything that was in the field before and add the new entry, seperated by a comma.
Let me know if you have anymore questions.
Will