An interesting and related story: European newspaper couriers used 6 wheeled Citroens to transport the paper across countries overnight at nearly 100 mph.
It's worth noting, that the Tissier modifications extended the hydropneumatic suspension to the extra axis. So despite being a flatbed or newspaper truck, they were still very comfortable.
I have seen a Tissier modification of a DS myself.
I mean, duh. You have to have suspension on the extra axle (well you technically don't but there would be some significant downsides to that approach) and just duplicating whatever the OEM did is the obvious way to go if practical. I'm not well versed in Citroen's hydraulic system of the era but I bet it would be only slightly more complex than adding a tag axle on airbags.
It is quite a complicated system, particularly because it was unique to Citroën. Only a few one-offs from other manufactures used the suspension system. So getting parts was not particularly easy.
Effectively, you ended up making them yourselves. You'll also need to ensure that the pump could handle the extra axle, and that there was sufficient hydraulic liquid to handle the now much larger hydraulic system. Remember the hydraulics also controlled the braking and the steering. All in the same system as the suspension.
There is good reason why a lot of heavy modifications of Citroëns of the era simply did away with the original suspension, and replaced it with bog standard suspension. It requires quite a bit of expertise to understand. It's possible, as Tissier proved. And Jerry Hathaway, who modified a Citroën SM do over 320 km/h, build a pickup out to carry it out of another SM.[0] I had the good fortune of meeting him back in 2017, and his SM.[1]
I have seen a Tissier modification of a DS myself.
Edit: Found the picture I took back in 2014: https://i.imgur.com/jKTv3od.jpg