While you could do exactly what you are describing, I'd recommend you NOT put multiple entries into the same field. Instead set up a Portal of related records. "Creating portals to hold related records" in your on line help is a good place to start.
If you insist on your approach, you could use:
Insert Calculated Result[Yourtable::YourField, Yourtable::YourField & "¶new txt to be added goes here"]