Extract the range of images to a folder using
KaoriMIM, not forgetting to offset the numbering to a free slot in one of your active mon files.
EG: free slot 3001-4000 in Mon2 and you want one of the new mobs located at 7001-8000 in its folder...
Make 2 empty folders, call one Mon2, the other 3001+ (for example)
Using KaoriMIM extract all of Mon2 into the empty Mon2 folder, then extract the range 7001-8000 from the new folder into the 3001+ empty folder, when it asks you for the offset or renumbering value put in -4000 (the difference between 7001 and 3001). Then simply merge the images into one folder and copy/paste the image detail lines from 0000 text file in 3001+ folder into the 0000 text file within the Mon2 folder.
Finally use KaoriMIM to make a new Mon file, name it Mon2 and select the folder Mon2 (or whichever you merged the 2 folders to).
Hope this is clear because It's 5.20ish am and I'm half asleep