While we recommend using our built in Sketch feature, you're more then welcome to use another sketch program you're more familiar with. As you finish your appraisal, you'll be able to upload your existing sketch file in the Summary section, under Addendums. Once you've reached the Addendum sub-section, scroll to the bottom and select New Addendum.
Note: Prior to uploading your own sketch file, be sure to export the file from your existing sketch program as a (.pdf) file.
Please see our article on adding addendums for more information.