Can I manually add in survey responses for people who have taken the survey offline?

Yes this is possible, but you need to make sure that your survey allows both invited and anonymous responses.

Answer your survey manually, and then clear your cache and cookies after you're done.

To clear your cache:

For Internet Explorer:

1. Go to Tools | Internet Options | Browsing History
2. Click Delete button
3. Check the boxes for Cookies and Temporary Internet Files
4. Click Delete Button
5. Click OK

For Firefox:

1. Go to Tools | Clear Recent History
2. Check the boxes for Cache and Cookies
3. Click Clear Now

Once you're done clearing your cache, you can add the next survey. Just repeat the process of filling out the survey and clearing your cache until you're done.