Following is a SQL statement which allows you to append data to a field (via the double pipe || symbol) rather than overwriting the existing data:

UPDATE YourTable SET YourColumn = YourColumn || ' Appended Data' where RecordID=x;

You could of course also use this to add data prior to the existing data:

update YourTable SET YourColumn ='Pre-pended Data '||YourColumn where RecordID=x;