// buildSrc/src/main/kotlin/CustomProjectVersion.kt
abstract class CustomProjectVersion {
    abstract val base: Property<String>
    abstract val qualifier: Property<String>
    override fun toString(): String = "${base.get()}${qualifier.get()}"
}

// buildSrc/src/main/kotlin/version-manager.gradle.kts
val customProjectVersion = extensions.create("customProjectVersion", CustomProjectVersion::class)

customProjectVersion.base.convention(provider { rootProject.version.toString() })
customProjectVersion.qualifier.convention("")

version = customProjectVersion

// my-subproject/build.gradle.kts
plugins {
    `version-manager`
}

// override the default
customProjectVersion.qualifier = "-custom"