Universal gameport to USB adapters that I've seen do not convert digital pads and joysticks (Sidewinders, Gravis, etc.), only analog pads and sticks (older 4 or less button joysticks and pads).
The adapters that come with 'USB capable' Microsoft gamepads are nothing but four wires and two plugs. The gamepads and sticks that come with these have USB inside them, the adapter just lets it have the right plug. Which is why it states in the manual that these will not work with any other joystick than the one it came with, might even damage the joystick or your computer if you do try.
Options for laptops: I've seen PC-Cards that add Joystick ports. Also, some laptops have a joystick port built in, but have a proprietary plug for it, so that an adapter is needed.