Add a custom dictionary to manageable dictionaries?
I'll answer but with a question at the end.
First of all you need jail broken device or any other way to access system files.
On your browser press on some word and select 'define' then 'manage'
Install some other dictionary and then you need file system access to navigate to:
/var/mobile/Library/Assets/com_apple_MobileAsset_DictionaryServices_dictionary2

There you will see several folders, enter inside and then once again inside assetdata folder, there you will see name of the dictionary that you installed previously. Enter inside that folder and then inside 'contents'.
Basically this is the folder of the dictionary and all the data.

Now on your pc search on the net for OS X dictionary files (they will be in zip file) extract from that file only the files of 'contents' folder and replace the files on your mobile idevice.

I did it to add Hebrew language dictionary.