Is there a way to mix AndroidX and a subproject that uses the support library?

All modules used in your app need to move to AndroidX together. You'd need to move your module you want to keep on Support Library into a new project and only use it as a maven dependency or AAR in your AndroidX project.

If you use multi-module project with androidX^[About] and want to use a module with support library you can try to create gradle.properties file inside module folder and define

android.useAndroidX=false
android.enableJetifier=false

Moreover I would recommend you to create gradle.properties for every module to configure your project

[AndroidX and support compatibility]