No, I don't think you have to have certain "credentials" in order to write an economics book, as with any profession and book writing, you just have to have knowledge and experience. When I buy an econ book do I look at where the author went to school or what he's doing now...not really. Although if it is a good econ book I might google the economist to learn more about him and to see if he's written any other books. Basically, WRITE AWAY and make sure to send me a copy, hahaha! Seriously, if you want anyone to read it I'd gladly take a look at it.
P.S. I admit, Armchair Economist is good, and if you like that book I HIGHLY recommend "The Undercover Economist" by Tim Harford. He has a column in the FT (Financial Times) where everyday people write to him asking him his views and opinions on everyday problems (dating, losing that sock in the dryer, whether or not to quit a job, etC) but he answers them with economics!
Here's the link... http://www.timharford.com/deareconomist/
I've emailed him a couple of times asking for career advice and such so you might want to take your question upon him and see what he has to say! Good luck with that book!