Yes you can do this, and I do it myself. go to your Nutrition Page and click on 'change diet settings' (in the drop-down menu options for 'tracker options' just under the date at the top. You will see "add a nutrient to track " - you can do it there. Make sure that you save your changes before you leave the page. ALSO, be aware that some bought foods contain fibre but don't show it in the Nutrition Label!
Good luck and happy tracking :-)