Michael Quinion, Ologies and Isms: Word Beginnings and Endings (2003) says that the prefix en- for forming verbs and the suffix -en for forming verbs come from very different sources:
en- Also em-. Forming verbs. [French, from Latin in-.]
The prefix can be added to nouns, adjectives, and verbs. Em- is a variant used before the the consonants b, m, and p.
One set suggests putting something into or on another; others have a sense of confining or restricting, or of surrounding something or placing it within something. A large group has a broad sense of 'put into some state or condition'. Rarely, it can suggest going into or on to something. Some are figurative terms derived from French or Latin roots, in which the origin of the meaning is no longer easily recognizable.
-en Forming verbs from nouns and adjectives. [Old English -nian, of Germanic origin.]
Such verbs have the sense of creating, developing, or intensifying the state suggested in the noun or adjective from which they derive. ... The ending is not active in the language.
[Note: Throughout the preceding extract, I have removed the author's numerous example words; to see them, you'll have to buy the book.]
A word like enrichen thus exhibits not only mixed parentage but an excess of verb-forming attachments. In general (and in the absence of guidance from The Simpsons) it's probably a good idea in forming new verbs to commit either to embig or to biggen, but not to embiggen.
UPDATE (9/26/14): In looking over this answer, I noticed a link in the LINKED column of the page to an earlier question, Verb form of "to blacken" versus "to brown". In his answer to that question, John Lawler points out several words that, like enrichen, have paired en-/em- prefixes and -en suffixes: embolden, enhearten, enlighten, and enliven. Readers intrigued by the question about en- and -en asked here will probably find that answer interesting, too.