How do I add a tuning potentiometer to modify the incoming control voltage of an XR8038?
You are aware (I presume) that this will not make a suitable VCO for an analogue synth (without jumping thru several hoops of complication). An analogue synth VCO MUST have this: -

Any single octave change in frequency comes about when the input voltage changes the same amount as for any other octave change. 

Rephrasing, if 1V produces 100Hz and 2V produces 200Hz then 3V should produce 400Hz and 4V should produce 800Hz. It's called a log-VCO in the industry and it's very important if you want to run VCOs in tandem and it's most noticable when the tandem VCOs are creating a root musical note and (say) a musical fifth. This could be brought about by (say) 1V on one VCO input and 1.5 volts on the other. If you then moved both control voltages up by an amount to create a rising pitch, the relationship musically between the two notes remains the same. You could end up with 3V on input 1 and 3.5V on input 2 - the two notes would still be musically one-fifth apart.

An XR8038 cannot do this without a significant amount of design work in exactly the area you want information on BUT, if you just want to try it out, connect a 1k pot across the supply and attach the wiper to the FM input via a 2k2 and see how it works out. This is called a linear VCO.