AppleScript : add to plist instead of replacing
I figured out that I need to add " at end with properties" and make the key's name different, in my case I'm adding the time in the key's name

tell application "System Events"
    tell property list items of property list file Secondplistfile_path
        make new property list item at end with properties {kind:string, name:"OverTime stoped at " &amp; hoursInNumber &amp; ":" &amp; minutesInNumber, value:myGoodNumbers}
    end tell
end tell