The only way I can think of is somewhat convoluted.
Go into your Wave Action, Frame 1 (or wherever wave starts) then in the 3D window select all arm bones affected by this action and press the “copy pose” button. Then change to the Walk Action, move to the appropriate frame if necessary and press the “paste pose” button.
You’ll need to do this for each keyframe so it will be easier if your keys are aligned, otherwise it could get a bit messy and take some tweaking afterwards Also, make sure these bones aren’t already keyed in different positions on nearby frames in the Walk Action.
If the wave action is pretty simple you might be happy with this approach. Hopefully someone has a better way though. It would be good if you could just box select keys and then copy/paste in the Action Editor directly, or combine NLA strips and export back to an Action.