وعليكم السلام ورحمة الله وبركاته
Nafl prayers are prayed just like Fard/Sunnah prayers. (Unlike Fard however, you'll also recite a Surah after Al-Fatiha in the third and fourth rakaat similar to Sunnah prayers.)
The intention is different, that's all.
So yes, you can recite any Surahs.