Annotation-processing on Kotlin #Serializable residing in external jar - android-studio

Question

I'm using kotlinx.serialization to obtain for example Data.serialize() according to this:
#Serializable
data class Data(val testField: String)
This works for a #Serializable class in the same project. I can use Data.serializer().
However, I also have a similar basic data class in an external jar from which I also want kotlinx.serialization to generate. But no corresponding .serializer() is generated for it. How should I configure Gradle for this?
project build.gradle
buildscript {
ext.kotlin_version = '1.3.60'
repositories {
mavenLocal()
google()
jcenter()
}
dependencies {
classpath 'com.android.tools.build:gradle:3.5.2'
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
classpath "org.jetbrains.kotlin:kotlin-serialization:$kotlin_version"
}
}
module build.gradle
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
apply plugin: 'kotlinx-serialization'
apply plugin: 'kotlin-kapt'
dependencies {
implementation 'org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.14.0'
...
tried the following
kapt files('path/to/repository/com/external/jar-name.jar')
or
kapt files('path/to/repository/com/external/jar-name-sources.jar')
or
kapt 'com.external:jar-name:version'
or
kapt 'com.external:jar-name-sources:version'
}
Do I need to somehow add the source jar of the external library to AndroidStudio to be able to do this?