Can I get more overrun despite single-speed machine?
For dairy based frozen dessert try incorporating more air prior to adding the mixture to your machine. Remember cold is your friend, freeze a mixing bowl then using a mixer, whisk or stick blender beat the mixture like you are making whipped cream. You should get additional volume, then churn as usual.
I’m not sure if this would work, but you may be able to whip additional air into the mixture after it’s finished freezing. You would need to work quickly, with very cold implements that could handle the resistance.