If you're running iOS 11 on your iPad, you can drag handwriting from GoodNotes to other apps.
1. Select the lasso tool in the toolbar
2. Draw a circle around the handwritten notes you want to drag
3. Tap and hold on the selection and wait until the selection gets "lifted" from the page background.
4. Move your finger and drag the handwriting to another app
The handwriting will either be converted to typed text automatically in apps like iA Writer or BearNotes or will be added as an image. It depends on which file formats are accepted by the receiving app.
If you want to add the handwriting as text, you can convert it with the lasso tool first by choosing on "Convert" after tapping on the lasso tool selection and then dragging the converted text from the pop-up to another app.