I see, what you're meaning, but the problem is that you can't extract separate instruments from a mixed (or already mastered file). However if the part you're talking about is singing only you can try to mix in your recorded guitar. If there is some kind of subby SFX in that part, you can try to suppress it using a dynamic EQ or multiband compressor on lows (you still might affect the vocals though), but if the whole band is playing, you can't filter the vocals out. So ask somebody to record the vocals and then mix them the way you wish. If you try to put the mastered song in background then it will sound messy, however you can eliminate both high and low frequencies with hipass and lowpass filters, make the sound quieter, add a reverb and put it in background but then the vocals will not be audible.