At the moment is not possible to add data manually directly in StepsApp (this will be possible in future), but you can add steps in Google Fit and then import them.

To manually add data, please follow these steps:

• Open Google Fit
• Click on "+" on the bottom right corner
• Tap on “Add Activity”
• Add steps/data
   
• Open StepsApp
• Go to “Settings”
• Tap on “Import from Google Fit”
• Import Now