Yes and no. You can’t use regular markers. But you can use what Blender calls “Pose Markers”. Here is how. Select your characters armature where you pose it. Then create a pose library entry. Go the the next pose on that character you want to mark, create another entry in your pose library. In the dope sheet action editor, under marker, select pose markers. Each pose you created now shows up as a marker on the time line where you put it. Repeat for each character. If you select another character, those poses will show up in the time line where you put them in the action editor.