List column? do you mean drop-down box? Are you saying that you have a data element in your main data source that is just a text type, you are putting it on the canvas, changing it to a dropdown list box, and then connecting it to your secondary data source to display the options?
So, that dropdown can only show items from the secondary data source. I don't quite understand what you are trying to do. Why would you want to change that data to something not in the dropdown? that doesn't seem to make sense. If you want it to be some other data, then you wouldn't want to use that external data in a dropdown anyway. What's the real scenario here? You can easily change the data in your own text box, but you'd not want to associate it to a control that is populated from a secondary data source. You'd want to have a separate regular text box where you type in what you want.