Carl Parker writes about it on Android Authority:
(partly reproduced here in case the original post goes down)
For Apps Installed on the Device
- Assuming you have installed the app on your device, go to the app’s
Google Play Store link and take note of the words after “?id=” and
ignore the rest.
- Connect your device to the computer via USB
- Open the command prompt on your computer and type the following
- Type the command
ls appfilename* (where “appfilename” is the app’s ID on Google Play
Store; make sure you include the asterisk at the end). This command
will display the app’s complete APK filename.
- Enter the following command:
mv apk_full_filename_here /system/app/apk_full_filename_here
- The device will now reboot. Your app is now saved as a system
For Apps Whose APKs Are on the PC Hard Drive
- Open a command prompt on your computer and navigate to where the APK
file is located.
- Enable USB debugging on your device and connect your
device to the computer via USB cable.
- Enter the following commands:
adb push apk-filename-here /system/app/
adb shell chmod 644 /system/app/apk-filename-here
Your phone will automatically reboot. Your app will now be saved as a system app.