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

How to add your steps and distance manually in Apple Health:

• Open Apple Health
• Tap on “Steps”
• Tap on "Add Data" (top right corner)
• Add steps

If you are already in sync, you should then resync with Apple Health:

• Open StepsApp
• Go to Settings
• Tap on Apple Health
• Resync Apple Health Sync

Otherwise you can add some steps via the Export Data feature:

• Open StepsApp
• Go to Settings
• Scroll to Export Data
• Add Steps
• Tap Import Data