First, to make it more that 6'6" you must seek planning permission as thats the maximum legal height without it.
Then as suggested, you can screw trellis to it, screw some some battens to the posts and fit your trellis to that.
If you want to do this without planning permission then a way around it is to fit some stand alone posts (not connected to the fence) directly in front that the trellis is fitted to).
Do not think that you can simply screw/nail trellis to the top of the fence panels as the next really windy day will see it all come down, you must have vertical supports that extend down the posts for at least the hight of the trellis.
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