The Free Draw Tool is new in FontCreator 6.5.
It is available in both the Professional Edition and the Home Edition.
When the Free Draw tool is selected from the Drawing Toolbar the Free Draw dialogue appears and any existing contours are greyed out. The brush width can be changed on the dialogue with the spin controls. Just drag the brush to paint freely, or hold down the shift key to draw straight horizontal and vertical lines. The comparison toolbar may be useful to show different glyphs before and after the current glyph as you are drawing.
To see how the newly drawn strokes look in other glyphs, scroll back and forth using the Previous Glyph and Next Glyph arrows on the Drawing toolbar. If you are not satisfied with the result, click “Delete last stroke” on the Free Draw dialogue to remove each stroke in turn. When you're satisfied, click on “Convert to contours” to add the contour to the currently displayed glyph (contours cannot be added to composite glyphs). To cancel, just close the Free Draw dialogue with the close button.
The new contour will not be joined to existing contours, even if it was drawn crossing them. In the Professional Edition, "Get Union of Contours" can be used to join them. See this tutorial thread on joining contours using the Home Edition.