How can I add an Old English dictionary to Dictionary?
Adding dictionaries is possible.  Finding the one you want may not be.  See

http://clasquin-johnson.co.za/michel/mac-os-x-dictionaries/index.html

For info on creating dictionaries for dictionary.app, see

How can I create a dictionary for Mac OS X?

How to add wiktionary as a source to the default dictionary.app?