How to make any sensor a wireless one

You may want to consider using RF transmitter/receivers that have an onboard ADC, such as the XBee series (available from https://www.sparkfun.com/categories/111). If all you want is to ADC the analog signal and transmit it, this may be your easiest option.

It is very common to take an analog signal, pass it through and ADC, into a microcontoller, and out a digital radio. There are many wireless microcontrollers with integrated radios and ADCs for just this task. One example is the mc13224v that's on an Econotag.

The other option would be to transmit the analog signal directly with AM or FM.