I don't know why everyone here is saying you can't do it.  I do it with every paycheck.  I have Quickbooks Desktop.  No you can't change amounts, but you can add customers and classes after the fact.  I have been doing it for the last 14 years!   Open the paycheck, then click on "Paycheck detail"  Don't change any $$ amounts, but you can change the number of hours (as long as the total matches what was originally there).  You can add lines, customers and/or classes.

I have heard that you can't do this with Quickbooks Online, which is why I'm dragging my feet to go there.