Yes! You may add steps as much as 6 months in the past on the site, but any steps you add before your actual registration date aren't counted toward your average, score, or diligence. They do show up on your graphs, however. These steps also won't show up on your step widget.

To add past steps:

Go to Add Steps. Scroll to the bottom of the page and click on the link "Add steps from previous days" (see screen shot below). From that page, open the drop down box to select the date you'd like to add steps to, enter your steps, and post your entry.