Within the Fitbit app, you can manually add an exercise like Walk and backdate the date to the appropriate day. Along with date and duration of the walk, you can enter the number of miles. Each mile adds ~2k steps to your daily total depending upon your height/stride length. So, the functionality you're looking for is essentially already there. While this updates the app, I can't speak as to whether it updates the weekly report as my guess would be that report is a static snapshot.
