How to add a Windows Phone 8.1 app version(xap) to my existing Windows Phone 8 app on store?

Yes you can.

When you are ready to upload your 8.1 package, click Add New in the package screen instead of the 'Replace' button (Replace will overwrite the current 8.0 build).

You will then have two packages for the app, one targeting WP8.1 and the other targeting WP8. DOwnloaders will get the version that matches their phone's OS version.

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I'm a self-taught noob amateur. Please take this into account when responding to my posts or when taking advice from me.