The standard 5200 controllers are based on two pots that go from roughly 10k to 500k ohms of resistance. However the 2600 paddle controllers also have pots of this value which means you can get into all sorts of mischief.
Many people have added a standard 2600 paddle grip to a 5200 controller.
Here are some links:
I actually haven't built one myself yet because I like the feel of the 2600 paddles and would rather build an adapter to use them. One way is described below (from an old post), other people have connected them up to wico y-controller cables.
You can make a little breakout box with a 5200 controller cable and Dsub15 socket. Wire it up first as a pass through and verify that a regular 5200 controller plugs into the box and works normally.
Then add a dsub9 socket and wire it up for a 2600 player-one paddle. Fire, ground, and pot common can be wired in parallel to the 5200 controller. Put in a switch to select either the paddle pot or the 5200 controller pot signal.
The only gotcha is the 2600 paddle moves in the reverse direction from what you want. You can open it up and solder the wire to the other side of the pot (leave the center terminal alone), or mount a switch in the bottom of the paddle to select either 2600 or 5200 mode.
I like this because the 2600 paddles are more comfortable in my hand and you don't need the second fire button anyway. In fact my box also has a masterplay-type circuit in it for genesis/2600_sticks. However, that's another story.