I didn't see this info here and coudn't find anything by searching for "overdraft" - but on their benefits page Flying Blue lists a possible miles overdraft for reward bookings.
https://www.flyingblue.com/en/progra.../tier-benefits



50.000 miles for Platinum members, 100.000 miles for Ultimate members.
In order to qualify Platinums need to have earned 300XP in the previous or current membership period.
You repay later by earning miles on future flights.

Anyway I have earned more than 300XP last year and don't see the overdraft working. Currently around 11.000 miles on my account, but the system wouldn't let me book a 30.000 miles award ticket - "you don't have enough miles" ... I wonder whether PFL are excluded (became PFL this January) or if you need to call in to request the usage of the overdraft.