Hi Him,
Business users who are not workflow-savvy often ask for this type of popup, but I never give it to them. 
Consider:
- the workitem will always be forwarded to one single user (instead of all responsible users) and that user may be absent
- you would need to build in all kinds of validations to ensure they don't choose inappropriate agents (like themselves, someone who is not allowed to approve, has already approved, etc)
- this kind of design facilitates corruption & collusion (they get to choose who validates their decision).
Instead, I recommend you simply ask the users how they will decide who to choose next, and build that logic into the workflow rules yourself.
Or at least, have that discussion with them.
cheers
Paul