Definitely possible.
Website speed optimization programs will even tell you that you should always do it, though there can be ease-of-use advantages to having separate files for discrete mods.
If a mod's styles extend or modify a distinct area of the main stylesheet, it would be good to merge it in that location so as to avoid overlapping or conflicting declarations; otherwise, you can always just add the contents of a mod stylesheet to the end of the main stylesheet.




