Editing Animations in Blender?

Hi, I'm new to unity and I've just started started putting animated characters into my game. I've done this with a construction worker and with a nice samurai I found. My plan was to take their characters, tweak their animations and then remodel them for my own game (if that sounds like theft then don't worry I'm not making money here).

What I want to know is whether I can edit the animations of these characters in Blender. I know I can make new animations, but whenever I import a character (single .fbx file) I can't find their animations on Blender's dope sheet or action editor.

If there's no way to simply tweak these existing animations like I planned, then could someone just point me in the direction of what I should be doing instead.

Thank you for your time.