You mean the one we use in the TOP for high frequency? The one generally placed above the mid-bass 12-15" drivers? If yes, then I wouldn't recommend that as that will produce just high frequencies. Better to get/use any satellite speakers or you can use speakers comes with your old 2.1 PC speakers if spare.
I do not know much about the horn speaker types but I am sure they are not designed for HT application.
Typically, a pair of satellite speakers similar to the ones you are using for your surrounds would be a better choice for Atmos. There are different type of ceiling mounts available to hang satellite speakers on the ceiling as long as the speakers have a built-in provision for mounting like a thread hole or key hole.
The satellite speakers in 2.1 are just tinny, and not capable of handling even 4-5 watts, whereas Atmos speakers need to throw sound upwards, to be reflected down and hence need sturdy, heavy magnet speakers.
On a side note, does your system has connections for Atmos? If so, find out what ratings/specs/impedance it needs, only then you can think about.
I guess it depends, I know some people who mounted the satellite speakers for Atmos on the roof, keeping the face downwards, also Atmos speakers are not always active sounding (even less than surrounds) so I think it should work.