Not true K_K.
All you need to do is enable the right things with BinHexEdit and make the turret a ROTGUNTURRET type mesh with 2 Rebel Lasers on it (all flyable ships with turrets use Rebel Lasers, I tried using Imperial but they changed to Rebel in-game).
I've made a Fighter (an ugly sucker but it works), it has 3 forward firing ImpLas, 2 forward firing IonCann, and a rear turret armed with 2 RebLas, you can use the 'F' key to tell the turret to fire at things, but you can't man it yourself because I dont have a seperate Opt for the turret.
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"No no son, you gotta hit 'em in the head so they stay down, like this! *WHACK*"
TK's OPT Factory (
http://www.ozemail.com/~richtersg/)