@pracas
you need some intelligent circuit to read and convert the sensor data to a digital signal in order to send it over the serial port.
Instead of using an A/D converter and a multiplexer why not using a small arduino board? I.e the small RBBB (Really Bare Bones Board), http://moderndevice.com/RBBB_revB.shtml
Is there a reason not to use a microcontroller with digital and analog input ports.
In any case you need an external interface to get the signals into you computer.