Blender script to take each of your keyframes in your timeline/dope sheet and turn them into pose library poses. Since it scans one frame at a time, tweening is preserved.

keyframes_to_pose_library.py

import bpy

start = bpy.context.scene.frame_start
end = bpy.context.scene.frame_end

# Corrects the name so that every action starts on frame 1
# If anim starts on keyframe 1, enter 0
# If anim starts on keyframe 0, enter -1
# Otherwise type in start - 1
offset = start - 1

# Enter the scene name inbetween the ""
# You can see it at the very top right, to the left of RenderLayer
# By default it just says Scene so type that in the ""
scene_name = ""

for x in range(start, end + 1):
    bpy.data.scenes[scene_name].frame_set(x)
    bpy.context.view_layer.update()
    bpy.ops.poselib.pose_add(frame=(x))
    bpy.context.view_layer.update()
    bpy.types.ActionPoseMarkers.active_index = x
    bpy.ops.poselib.pose_rename(name="F" + str(x - offset) + " Pose", pose="Pose")