You can’t pull them from an “earlier” envelope (it’s not a new envelope each month), but adjusting the amount of the Fill does the same thing. For example, my Fuel budget is $300. At the end of the month I have $20 left, so my automatic Fill (with the Set instruction) adds $280. Then I remember to include a $15 fill up from 3/29, so my actual ending balance should have been $5. I can adjust this by changing the Fill to $295 so that April has enough.
Goodbudget treats everything like cash, so if you “past-date” a transaction it doesn’t adjust retroactively. I make it a habit to clean everything up on the last day of the month, but Fill adjustments are a simple fix if I do miss something.