Transfom the PWM in to a varying voltage (stabilised SMPS ) and then use a voltage to res converter (like a FET). Hope there is a much simple way. Like Nigel says, give more details!
You start saying PWM, and are now saying frequencies? Anyhow, if it is frequency, a system may be designed around a frequency to voltage chip and the floating voltage controlled resistor shown as figure 10 in this datasheet:
Those are very low frequencies though to be working with. If it is indeed PWM, I believe a lowpass filter will convert to relevant voltage levels. What response do you want anyhow, linear, logarithmic?
why exactly do you need a "variable resistor" are you trying to make a variable amplifier whose gain is controlled via PWM freq?
plus anyway you are not modulating the pulse in this case, it sounds more like FM
some info really is needed, what is the variable resistor for?