It would be tricky, but I think (think, mind, since I've never tried replacing a regular NPC with another regular NPC), that it would be a similar process as adding a totally new NPC. You'd have to do some scripting, changing dialogues, changing the checks for Disciple so he doesn't start acting like G0-T0 when he shouldn't (and changing G0-T0's checks so it doesn't start getting Disciplish).
I'm not sure about how you would go about changing the dialogue just so that a male Exile can hear Disciple's backstory although I have an idea about how that could be done...you could combine the male Exile Khoonda dialogue with the relevant parts of Disciple's regular dialogue--with some checks and scripts removed or changed--and change the Khoonda on enter script so that your new dialogue would play instead. Edit: This part of the script k_enter_601.ncs
AssignCommand(GetObjectByTag("disciple", 0), ActionStartConversation(GetFirstPC(), "disc_enc", 0, 0, 0, "", "", "", "", "", "", 0, 0xFFFFFFFF, 0xFFFFFFFF, 0));
Change disc_enc to your new dialogue name. I think that would do it. If it just made things really weird, you could try adding a male Exile branch to Disciple's regular dialogue...(sorry for being confusing, the heat is melting my brain!)
If you're seriously into modding K2's scripts,
http://www.team-gizka.org/downloads.html) has a link to all of the decompiled K2 scripts. It's on PC Game Mods, so it might be hard to get now, but I can send you the whole k_enter_601 if you need it.
Short answer, yes, I believe it's quite possible to replace G0-T0 with Disciple, but it would be involved. You would definitely need to track down G0-T0's recruit dialoge and change it, and change Disciple's as well, add another exclusively male branch to his initial dialoge and write a script that would put him in G0-T0's place...also, find and do something about the scene on Malachor V between G0-T0 and Remote...
Check the tutorial on recruiting a new NPC to start you out (
http://www.lucasforums.com/showthread.php?t=131944), if replacing G0-T0 is more enticing than getting Disciple to talk more to a male Exile. I'd offer my own K2 Dustil recruit mod to dissect, but I'm not quite finished with it yet.