Use the following Excel tool to to generate Spend and View Keys needed for Simplewallet import:
http://cdn.bitcoin2.network/Documents/B2N%20Simplewallet%20Keys%20Generator.xltm

You will need to open your GUI wallet, click on File (top left corner), and select Export key from the drop down menu. After that, copy the wallet key and paste it into the Excel sheet in the green box area. Click on "Generate Simplewallet Keys" button to create your Spend and View keys.

Now you can import your GUI wallet into Simplewallet!