Can I buy a 5-day metro travel pass at the Paris Porte Maillot subway station?
Are there cash desks or ticket vending machines there?
There will be both a staffed ticket windows and ticket vending machines at Porte Maillot Metro and RER stations. Not all entrances will have their ticket window open though (at the same time), so you may have to check other entrances if the one you're at happens to be closed.
You can purchase a 5-day Paris Visite ticket from the ticket vending machines or ticket windows.