Hello,
Sorry if I ask silly questions but I need to get my head clear on this matter. I have a fst4b transmitter and r6b receiver. Is it possible to add atleast one extra channel to my tx? I searched on Google but there arent many articles and I didnt understand much. Most people say that its possible but dont state how..
I hope someone would be able to help me 

Firstly i must say that i do not want to discourage you but just a word/phrase/stance of caution . Whatever you do with that transmitter do your reading understanding  and asking others do it deeply and if you plan to modify it be sure what you are doing. Because its the buck of horse which controls your model , and others safety.
Okay back to topic.
1 try to find what mcu is in the core of the radio ?
2. You can easyly make a 6 channel via arduino nano/pro mini/uno. Just see akhil 's super tx ct6b mod.

www.rcindia.org/self-designed-diy-and-college-projects/super-t6-mod-(custom-motherboard-and-firmware-using-arduino)/
here you go

I basically need only 4 channels but to change mode in multiwii, a 5th channel is required. So I did this -->
I connected the throttle on the rx to the mode on the multiwii. Then I connected it to the multiwii GUI. I wanted it to activate MAG so I came to the AUX1 section and I selected all three(high,mid and low) values for activation. I thought of this as the default values stay 1500 in the GUI and in flight(I hope). So the quad will atleast maintain altitude instead of falling suddenly... Will this work in flight cause now when I connect the board to the pc, the ftdi doesn't respond and only one light blinks instead of two. Also there is no Quadcopter image shown on the screen. But when I connect the battery to the quad and arm it, the motors work perfectly. I just needed the MAG to be activated in flight and so I wanted an extra channel. I don't want to mess with my radio. Can you tell me if this technique will work.. It would be great help.

Thats a beginner radio meant for planes  would suggest for rcb6i for you.
It has a 3pos switch and a 2 pos switch.

Ya but will the above work?

to be frank I dont understand above said

Considering you know about multiwii,
Now that I don't have a 5th channel and I want to self level mode which is ANGLE in multiwii GUI. Now instead of connecting the throttle on the rx to the throttle on the multiwii,I connected throttle on rx to mode on multiwii which is AUX1. Then I hooked up the board to GUI and I selected all three points for activation of self level mode. And when I removed my ftdi and put it back against showed self level mode on.
I just took it for a flight and self level is working well...
Admins please close this thread...