What are the limits of the Purrodaemon’s Weapon Steep ability?

The Daemon, Purrodaemon ability "Weapon Steep" states:

If a weapon remains sheathed in its body for at least 24 hours, the weapon absorbs some of its essence and gains magical enhancements. A purrodaemon can have up to a dozen weapons lodged in its body at a time, but only one can possess magical enhancements at a time. The total enhancements cannot exceed a +4 effective enhancement

Can this ability be used enhance an already magic weapon further? For example: Purrodaemon steeps a +1 Impact Guisarme. Can he then add up to a +4 enhancement to that weapon, such as Brilliant Energy? Or can he only add enhancements to mundane weapons?