Currently it is possible to add your own custom testset in your ~/.jprt.properties file, but it cannot override the default set of build flavors or build targets. Allowing this is pretty simple. The following is code in jprt.properties:

# Select build flavors and build targets
jprt.build.flavors=${my.is.hotspot.job ? ${my.build.flavors.hotspot} : ${my.build.flavors.default}}
jprt.build.targets=${my.is.hotspot.job ? ${my.build.targets.hotspot} : ${my.build.targets.default}}

It needs to be replaced with:

# hotspot testset has custom build flavors and build targets
my.jprt.testsetHasCustomBuildFlavors.hotspot=true
my.jprt.testsetHasCustomBuildTargets.hotspot=true

# determine if specified testset has custom build falvors or build targets
my.jprt.testsetHasCustomBuildFlavors=${my.jprt.testsetHasCustomBuildFlavors.${jprt.test.set}}
my.jprt.testsetHasCustomBuildTargets=${my.jprt.testsetHasCustomBuildTargets.${jprt.test.set}}

# Select build flavors and build targets based on specified testset
jprt.build.flavors=${my.jprt.testsetHasCustomBuildFlavors ? \
    ${my.build.flavors.${jprt.test.set}} : ${my.build.flavors.default}}
jprt.build.targets=${my.jprt.testsetHasCustomBuildTargets ? \
    ${my.build.targets.${jprt.test.set}} : ${my.build.targets.default}}