You can install both, and keep the existing Kobo English dictionaries, making three English dictionaries on board, you just have to sacrifice some of the other language dictionaries. For example, if don't want to keep the German and French dictionaries, then rename
.kobo/dict/dicthtml-de.zip to dicthtml-de.backup.zip
.kobo/dict/dicthtml-fr.zip to dicthtml-fr.backup.zip
Then copy the custom dictionaries into .kobo/dict and call them dicthtml-de.zip and dicthtml-fr.zip. Now selecting the German or French dictionary in the reader will actually give you the relevant custom English dictionary.
To go beyond that (add the custom dictionaries as new dictionaries and not replace existing ones) would probably need a change to the firmware.
Last edited by ShellShock; 11-20-2012 at 02:09 PM.