You can use the 6 button joypad if you make a custom cable for it, with EP connector. You need to isolate the three buttons commons(the ones of your choice). And you also must discard all the internal joystick electronics, because the EP commons are positive, instead of the GND used in common joysticks.
Observe this picture:
On
Zozo's page you can find information about the EP connectors, but I am going to repeat it here:
Control 1 / Control 2
6 láb széles, 12-ből 9 van használva. / 6 pins wide, 9 out of 12 pins used:
A1 : Keyboard J (Common) B1 : OV
A2 : Keyboard L B2 : Keyboard K
A3 : nc B3 : nc
A4 : KB4/9 (Right) B4 : +5V
A5 : KB2/7 (Down) B5 : KB3/8 (Left)
A6 : KB0/5 (Fire) B6 : KB1/6 (Up)
A perjel utáni számok a 2-es csatlakozóra vonatkoznak. / The numbers appearing after the slashes apply to control 2. 
The control pin positions are viewing the connectors from the rear. All EP EDGE connectors are named the same, with the connector in front of your eyes, numbers start at the left hand, B is the component side(up) and A is the solder side(down).
You will need the 4 direction leads and the fire lead, then the three commons(J, K and L). Discard the +5v and GND. You have enough with an 8 leads cable.
