Hello. I am designing an experiment where I need participants to respond to stimuli by typing words (and numbers higher than 9). Is it possible to programme such responses as valid?  E.g. so they type a word and then hit return in response to a stimulus.

Yes. You'll want to see the documentation for /validresponse, /correctresponse, /isvalidresponse, /iscorrectresponse as well as the /mask attribute.