All you need to do is move the apk from /data/app (or wherever it is) to system/app (not data/system just system/app) and make sure the permissions and owners match and reboot. Did this a lot on my old X2. Once its in /system/app and all the permissions and owners match the files around it it will be a system app. I use root explorer btw.
As for permissions, all my system apps are rw-r--r-- and the owners are both set to 0-root. Make sure to click the Mount R/W button at the top of root explorer to change it to Mount R/O before you try to make the changes.
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