So despite being a habitual lurker I thought I should post this. I've written a Python 2.7 script that allows you to decompile and recompile My Pokémon Ranch saves, built upon the research of suloku.
If you want to add new pokémon to the ranch just decompile the save and add the encrypted 138 .pkm file to the "Pokemon" subfolder of the extraction folder and recompile. Make sure it isn't named similarly to the other files in there or the script may make a terrible mistake. It'll use the trainer link information from the last ranch pokemon in the list.
It isn't fully featured, it only really handles pokémon dynamically.
Script is here: http://pastebin.com/NnjYjqYR
Hope someone finds it useful.
PSA: Python scripts can be dangerous, don't run unknown scripts from somebody you don't trust. Get someone who knows what they're doing to read through them first to check they don't do anything bad.