you have to configure the *.APP file. it is a text file for emus and apps that aos can launch. you can get this from the aos website then edit to your liking. there is a blank template for other applications you want to add. i think it has to be the same name as your emu exe, mine is vantage.app for vantage. the emu should already be properly working without aos.
then inside ArcadeOS menu, you can switch to that other emulator/app. aos requires that there is a dosmame emulator configured and present or it will exit back to dos with an error.