I think that depends on the user.
You should be able to add your nature to the susanoo. The reason that didn't happen is due to plot shield for Assura descendants. Imagine Madara's susanoo emitting hear or fire it would destroy and render mokuton useless.
That'd be awesome. A fiery susanoo.



I dont see why not except as a plot shield for Ashura descendants.

No but you can add elements as their weapon.
I dont think so. Sasuke already performed the highest level shape transformation plus affinity merging but still unable to create full lightning Susanoo. You still can see Susanoo's armor there
You must be registered for see images
If Sasuke can why he is not converting his Susanoo into Full lightning body Susanoo? Nothing held him so it clearly suggest that Susanoo exist as what it is even though its made from Chakra(User). At best the user only can make " something" wearing Susanoo's Armor without completely erase the Susanoo natural form.
Was it ever stated by Susanoo can't be converted that way? Despite being raw chakra?
It doesn't need to be stated but 'showing' act as a fact presenter and its better than words.
Like here we have Bugatti Veyron SS vs Koegnigsegg Agera R. On the paper and mouth words many people say Bugatti will win the drag race but in the race Agera win. So showing is more credible than word of mouth.
What I was implying was, if there is a reason. We know why wood element can't be absorbed by preta because wood is not necessarily chakra. Likewise is there a specific reason why Susanoo despite being chakra cannot be transformed into nature type of the wielder?