As you’ve discovered WordPress ignores rewrite rules added into your .htaccess file manually. Not many people actually know WordPress has in-built functions, a core rewriting class that allows you to add custom rewrite rules into your themes functions.php file.
You can read up on the WP_Rewrite class here and if you do a search on this site you will find numerous questions from people asking to do similar things.
I am a firm believer that writing and posting the code for you is cheating, but if you visit the aforementioned link and do a search you will find all the info you need to implement custom rewrite rules.
Of course if you get stuck, please post the code you do end up writing and I amongst others will gladly help you out and point you in the right direction.