In some cases, you will find it difficult to add new fragments to your molecule file, for example if you already have structures cleaned in 3D.
To add a new fragment to the canvas:
Choose the New substituent option from the Insert menu.
Draw the structure in the new canvas. You can perform a 3D clean ( Structure/Clean 3D ) on the new structure to match to your original 3D molecule.
Click the Transfer button in the top left corner to return to the original canvas and place the new fragment.