This mod allows you to ally with the grox without worrying about the entire galaxy hating you for it. It works if you install it before, or after allying with them, at least for me.
Download the mod from here and place it inside the Data folder in your Spore directory:
C:\Origin Games\SPORE\Galactic Adventures\Data
If you have the CD version, you can simply download the original version of the mod, which you can find here: http://www.sporebase.com/forum/index.php?topic=5693.0 just follow the instructions and you should be good to go. I am not sure about the steam version of spore, so just try both of them and see what works?