Lennard,
Yes, you can do that.
If you want three gallons of 5% that would probably benefit you your wallet too.

Start with whatever oil blend you want, say 20% would be 25.6oz of Klotz oil or equivilant, then add 102.4 oz of methanol.

Mixing the two together will yield, like you said a 7.5% nitro mix, and 18.5% total oil mix.

making two gallons of 0% and adding the gallon of CP15% will yield you 3 gallons of 5% nitro, 19% oil mix.

don't worry about oil compatability, the klotz and CP oils are completly comaptable even though they are slightly different.

Fuelman