In response to my own question, here is what the macheist support people told me:
I believe you can go to http://evernote.com/partner/retail/ and enter your code, then select Existing Account and click the Premium button to add it to your existing account, but you should verify with Evernote customer support to be sure.
Update: It worked! I got two Evernote activation codes after buying the bundle (one for 12 months of premium and one for 3 months). I entered them one at a time on the website[evernote.com] and chose "login to existing account". My Evernote account is now showing premium service until Jan 17, 2014!
$29 for $60 of Evernote is well worth it, best deal I've seen on Evernote. Thanks and repped!
side note: I wonder if I could buy two bundles and extend my Evernote premium out to 30 months?!
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