Hi @Tony_A
There are pros and cons here for you to consider So I will outline them for your consideration.
Right now, I am looking at the same project myself re cladding.
1 It is super easy and very tempting to attach things to the fence as is in your case just screw into the top middle and bottom rails and the jobs done.
If the post is in the way of where you want the panelling sitting further out from the rails you may want to add your own rails to the same level as the posts, so it is easier to attach the cladding to.
2. In 20 years' time and that fence getting old and maybe time to replace it anything attached to the fence will need to be pulled off and put back on so it's a ticking time bomb for a future double project Fence and cladding rebuild. I have been there and done that not fun.
As in option 1. you will need to add rails for an even surface to screw into so why not add posts and make a free-standing structure just Infront of the fence? Costs the posts and some cement but it will never have to come down.
If you want cheap strong posts use colour bond fencing the posts $ 15 ea used back-to-back C or to a 50mm x 50mm end post (they are inexpensive and paintable) that is 50mm wide and will allow wooden rails or colour bond rails $15ea to slot into the C section. Timber easier to screw too or just use self-drilling metal screws.
I am opting for option 2 separate panel posts and rails personally.
But feel free to use the fence I did in my early days of yard landscaping.
