You need enter text input mode first. You can then move the cursor to the desired location of an existing line or create a new line. Now you switch back to handwriting to text mode to write. All recognized text will be put on this line. This works with existing text lines or new text lines.
For a text box, it is also possible to edit, and continue to use handwriting to text in the same text box later. To do this, you need: 1). enter handwriting to text mode; 2). long press the text box to be edited until edit options pop out. 3). move cursor to the desired location; 4) write. The text will be appended in the text box, not in a new text box.
If you are in a text box, your writing should append to it unless you exit handwriting to text mode, or you create a new text box.