While I'm certain there should be a way to achieve this I think that this is a very bad idea. Because you're using sprints and the term velocity I'm assuming your using an agile methodology.
Agile focusses on the responsibilities as a team, not as an individual. While someone might be helping others all sprint long getting their work done and making sure the sprint goal is reached, he/she won't have any issues assigned to their name and thus it looks like they've performed badly even though they've fulfilled a very important role in that sprint.
Just my 2 cents.
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