How do I use a javascript confirmation box when a button is clicked that also has causevalidation=true?

I tried:
saveIt.Attributes.Add("onClick", "return confirm('Are you sure you want to save?');")
update.Attributes.Add("onClick", "return confirm('Are you sure you want to update?');")

However, both button already have onClick events because of the causevalidation=true property. Is there a way I can add this attributes.add to the exsisting onClick event?

James