Beeing a console player, it was hard form me to play MAME and NES game from a keyboard. So I came with this idea to add a joypad to my keyboard.
Here is what zou need to do this:
an old working keyboard
a joypad ( I used a Playstation one joypad)
soldering gun

Step 1: Open the Keyboard

Open your keyboard and and analize the circuit board. Your keyboard circuit will look something like the one in the picture. It is posible that the circuit ends (1 – 14) are not placed identical to this, but that is not a problem because I will explain the principle behind this.

Step 2: What Keys You Want to Use

Decide what keys you want to use for controls. I picked the following keys:

UP    – I
DOWN                 – M
LEFT    – J
RIGHT – L
X – U
CIRCLE    – . (DOT)
TRIANGLE – , (COMMA)
SQUARE    – Y
START                – ]
SELECT – ....DONT REALY KNOW WHAT KEY THIS IS BUT IT INSERTS COIND :D
L1    – O
R1 – K

NOTE: Some games demand that you type OK to start. That is Why I used the O on L1 and the K on R1

             It is not so important what keys you choose because you can configure them inside the game. The reason I picked these keys is because I used the original Joypad cable wich only has 7 wires inside. This means that I can get a maximum of 12 combinations either by soldering 3 wires on the left side and 4 wires on the right side or the other way around. If you use a cable with more wires then you have more options in choosing your keys, they don't have to be so close together like mine. The keys I choosed are close togheter because they have common wires. The advantage of beeing close together is that you have room on the keyboard for another player.

Step 3: Finding the Right Circuit Ends

Find out witch circuit ends corespond to your keys. Here is what I mean:

           First you need to open a text document file while you have your opened keyboard attached to the PC. Take a wire and put an end to a number on the left and another to a number on the right. A character will apear on your screen. You should repeat this proces until you have discovered all the keys that you decided to use.
           Another way to discover the keys is to folow the circuit. For example you look where the letter U makes contact on the circuit sheet and you follow the two lines . One of the line will end up at a number on the left and one at the number on the right. If you make contact between those two numbers the letter U will apear on your screen.
          If your Keyboard circuit in not the same as this, then the best way to discover the keys is to follow the lines from where the key makes contact on the circuit sheet.
At the end of this step I have obtained the following result:
UP –     6-5
DOWN    –     5-9
LEFT –    5-7
RIGHT              –    7-7
X    –    5-5
CIRCLE –    7-9
TRIANGLE    –     6-9
SQUARE –     5-6
START             –     6-6
SELECT    –     7-6
L1 –     7-5
R1    –      6-7
Observing these combinations, the numbers on the left that I used are 5,6,7 and on the right 5, 6, 7, 9

Step 4: Solder the Wires to the Circuit Ends

Solder the 7 wires to the keyboard circuit. The colours of the 7 wires are:
Black (B)
Blue(Bl)
Brown (Br)
Green (G)
Red (R)
Yellow (Y)
Orange(O)

The order in witch I've soldered them is:
5 6 7 5 6 7 9
B Br Bl G R Y O

Step 5: The Joypad

       Open the Joypad and observe the circuit board. You will see that the place where the buttons push on the circuit is a square with many thin lines. When you push a button the rubber underneath presses on this spot and makes contact between the thin lines on the left and the ones on the right. So what you need to do is to solder a wire that makes contact with the thin lines on the left and a wire that make contact with the thin lines on the right. Here is my combination of wires.

UP    – 6-5                  -   Br-G
DOWN                         – 5-9    -   B-O
LEFT    – 5-7    -   B-Y
RIGHT – 7-7    -   Bl-Y
X                                   – 5-5                  -   B-G
CIRCLE                      – 7-9                  -   Bl-O
TRIANGLE                 – 6-9    -   Br-O
SQUARE                    – 5-6                  -   B-R
START    – 6-6    -   Br-R
SELECT – 7-6    -   Bl-R
L1    – 7-5    -   Bl-G
R1 – 6-7    -   Br-Y

I have put an electronic scheme that shows you how to solder the wires because in my pictures you don't see what realy happens.
When soldering the wires try to leave the front side as clear as you can so that the rubber can make a full contact with the circuit board.

Step 6: The Finished Product

After you"ve done this steps close the keyboard and the joypad.
Now you have a Keyboard with a Joypad that makes playing games much easier. You can now play on your PC, games that were adapted from console, without feeling a diference.

Have fun and enjoy.