If all the poses are on the same armature OR if all the poses are on identical armatures (meaning armatures with the same number of bones and with the same bone names), you can create a pose library with all the poses you want saved to it. Then you can attach this pose library to a project file containing the armature that you actually wish to animate, and all the saved poses will become available to it. The Pose Library is created and accessed from the Object Data tab of the Properties Panel (while an armature is selected), then under the Pose Library section).
To blend the poses during animation: Make sure you're in Pose Mode any time you are doing anything with the Pose Library. After choosing poses in the Pose library one by one (the poses become listed in the library when they are saved to it), you must click the "+" icon to save the pose to the library. Only selected bones are affected by any changes in pose that are brought about by using the pose library, so keep this in mind. You can keyframe any pose activated in the Pose Library (using the "magnifying glass" icon to apply the pose to the currently selected armature) using your standard workspace windows and editors as you normally would. Then you can choose another pose from the library at another frame, and keyframe again to get interpolation between the two poses.
If it turns out that the desired poses were originally from different armatures, this can all still be done, but extra steps are required. You would need to open the extra versions of the armatures, and for each one, select all its bones in Pose Mode, and then Ctrl + c to copy all the pose positions of all selected bones, then select a different armature -- the one you eventually want to animate -- in Object Mode, switch to Pose Mode, select all its bones, and Ctrl + v to paste those pose positions to this other armature, save the new pose to your pose library, and then continue with these steps until you have added all the needed poses to the pose library. Your pose library can be saved and appended to other files also, which can be handy. This way, you don't need all of your armatures in one file to get the pose library built. That said, it might actually be easier to append the armatures to the file containing the pose library instead of the other way around, as this might entail less chance of pose library corruption. So, you could append each armature to the file containing the pose library, add the pose of that armature to the pose library, save everything, and append a new armature to the file containing the pose library, then save the new pose to that library. This might be the preferred approach if none of your desired poses are on armatures that all exist within one single file. If they do, then simply add their poses to the pose library all in the same session, one at a time, and then get rid of the armatures you no longer need afterwards. You should end up with one armature and many available poses. Don't delete any of the other armatures until you know all your poses work.
