Is it possible to exchange two footmen for a knight anytime? For example, in an area without any castles or strongholds (no mustering points) can I at any time during the game exchange two footmen for a knight?
Absolutely not! Nowhere in the rules is this permitted (also it doesn't make any thematic sense :))
As per page 9 of the rule book, you may upgrade a footman to a knight for the cost of 1 muster point, but otherwise no. Note there is also no mechanism by which a player may willingly remove troops from the board (obviously you are forced to remove troops if you exceed your supply limits)