Sure you can do this. It can easily be accomplished by using notifications in the workflow and having a "Reviewers" group setup and make them available for the notifications. It isn't really a drop-down menu, more of a check box. The drawbacks are that you can process a workflow without selecting anyone, so you might want to have a default person be notified. You can also select multiple people for the notification.
The only other real way to do this is to create an add-in that processes with the workflow. The implementation might go much smoother, but the setup will be significantly more complicated.
Here is an option we offer our customers I call dynamic approval. We integrate an add-in that controls the approval workflow with the use of file card of the ECO. Here is brief demo of the feature. http://www.youtube.com/user/CAPUniversity#p/c/B26C9D4AA5F02477/2/3mbrzyASJ0E. If you have any questions about how it is implemented let me know.