From the FAQ on their website:
Is Silver Efex Pro 2 Smart Filter compatible?
Yes, Silver Efex Pro 2 is Smart Filter compatible. To use Silver Efex Pro 2 as a Smart Filter, please perform the following steps:
1. Open your image in Photoshop CS3 or later.
2. Select the layer that you would like to apply Silver Efex Pro 2 to.
3. Access the Filter menu, and select the option Convert for Smart Filters.
4. Access the Filter menu again, roll over Nik Software, and select Silver Efex Pro 2.
When using Silver Efex Pro 2 as a Smart Filter, you can go back and edit the effect applied to the image as long as that file is saved as a PSD or TIFF file with the
layers retained.