Yes, affix is a Scrabble word!
...and is worth 18 points in Scrabble, and 18 points in Words with Friends
affix
Verb
1. Attach to
2. Add to the very end
3. Attach or become attached to a stem word
Noun
1. A linguistic element added to a word to produce an inflected or derived form
What Scrabble words can I make with the letters in affix?
3 Letter Words
2 Letter Words