I've had a couple of questions on this, so here are the step-by-step instructions to do the conversion. I assume you have a "fresh" installation of DOSBOX on your machine -- ie, one that will give you a command prompt. I used DOSBOX 0.74, I am not sure if older versions will work.
(1) Put all your saves in a location; I will call this c:\xeen
(2) Copy the files from the MM4/5 install to another location; I will call this c:\xeen2
NOTE: By default, these files end up in "GOG Games\Might and Magic VI Limited Edition\Might and Magic 4-5". The files are NOT in "Program Files" or "Program Files (x86)" -- the old files are there. So, be sure you copied the right ones. You will know you have the right files if you have the files GAME1.inst and GAME2.inst both present, along with the file dosboxMM4-5_singleCD.conf (and several other files).
(3) Start up DOSBOX
(4) Run the following commands (in order) from the DOSBOX command prompt (assuming you are at z:\> prompt):
mount c c:\xeen
imgmount d "c:\xeen2\game1.inst" "c:\xeen2\game2.inst" -t iso
d:\game\sav2wox c: d:
Then follow the on-screen prompts.
(5) Finally, exit DOSBOX and copy the WOX files to the normal saved game folder. You can delete the folder c:\xeen2, but you may want to keep c:\xeen around for backup because it has your original SAV files as well as the WOX files. (The original SAV files are in the c:\xeen\BACKUP directory.)
Note that you do NOT need to download sav2wox, nor do you need to put it in c:\xeen, because it is present in the image file.