Jar-file in jar


i am building a shadowJar with johnrengelmans shadow plugin (com.github.jengelman.gradle.plugins:shadow:1.2.3)

i want to copy an additional jar-file (a.jar) into the resulting jar. only the plugin seems to merge the content of a.jar into the resulting jar. instead i need a.jar in a subfolder within the resulting jar.
(any other file gets copied correctly. only .jar files are getting merged)

how would one do that?

thanks for any hints