Is there a way to assign a Participant a set of students as reviewer's before the Participant has made that submittal?
No. The reviews are internally allocated per submission, not per user. So you need the submission first. In a future version, I plan to add a feature that would automatically pre-create empty submissions for all authors, so this would be possible. But not in current versions.
Is there a way to assign a Participant a set of students as reviewee's before the Participant has made that submittal?
Well, yes. As long as those being-reviewed students themselves have submitted something. The reviewers themselves do not need to submit anything.
set up the Workshop Participants with five students that will review them and five students that can be reviewed by them
As said above, this is not easy, if possible, to set up. You would need to temporarily switch the workshop (which might be hidden yet from regular students) to the submission phase, log in as each participant, submit an empty draft submission and then allocate it for assessment. Once that is done, you can show the workshop to students. As long as the workshop remains in the submission phase, they can edit the pre-created submission records.