Do you have to earn miles or can you use miles to extend expiration as well? Usually other programs any activity counts (earn/redeem), but the terms on F9 do say "earn".

I have been searching the forum and cannot find confirmation of this.

The easiest would be to use points.com for a small transfer or use magsformiles.com which has Frontier options.

I'd also like to know how quickly the activity is generated - I have some expiring in 5 days so would need activity to be logged by then.

update: this site covers it better and mentioned that it has to be EARNING activity, not redeeming. But, you could at least try redeeming your miles soon before expiration to get some value out of them.