Yes, you can compel expert witness testimony, but this is different. Testimony is answering questions. Here, they are seeking to compel engineering/coding work. I've never seen that compelled by a subpoena.
If you read old-ish legal journal articles about expert witness compensation, you find that the requirement to do up-front work in order to generate the knowledge required to handle questions is a dividing line for whether (or, at least, whether in the 1960s) expert testimony must be compensated. From that, I gather that this kind of request isn't unprecedented.
