Doubling the same poses might require more attention while setting up the animation, as doubling the entries with similar information, might result in adding the movement of the mesh.
Let’s start with the fastest way, which might clutter your object manager and increase perhaps problems later on (copy and paste a character).
The simplest way would be to use the Object Manager> Object> Current State To Object [CSTO] command while the object with the desired pose is active.
Then drag that new object in editing mode into the PoseMorph, leave it as relative.
You can set the new object created with CSTO to invisible for Editing and Render.
Internally, you select the two (or more Poses) and right-mouse-click [RMC] on one while active. Use the copy, then the paste option. After that, select the new copies and RMC to get the flyout menu again, to use the Merge command. Rename it perhaps.
A good idea is to place the PoseMorph in question to be merged, in a folder, so it is clear which newly created pose will work as those, so even after a long time, the use of it is apparent.
Here is a one minute screen capture
There is one gotcha, if the Mixing of the merged poses was previously not identical to the point, it can’t work as before. If one was Relative and the other one Rotational, this will not survive the merge process.
Here is a short illustration of the problem
As shown, fi the rotational axis is the same, from all merged Poses, this can be fixed.
All the best
Just for reference