Modification Swapping knees: single->Double

Well I have not done much of this personally, but I have a couple thoughts. First of all, I think adding torso joints may be easier than you think, depending on what kind of modifications frighten you. you can divide the torso if you have a rotary tool to carefully saw it apart, then you could create the shelf/lip features on the separated pieces in order to make it function like intended. I think all in all it's probably a bit easier to get dramatic results on this type of mod without extensive skills in creating joints.
When it comes to wanting to convert a single joint to a double joint the problem is that single joints are extremely often just attached to the limb itself and not a seperate piece like double joints. So to replace the joint itself you have to remove it and then hopefully you can make it find what you need that fits in there and also functions completely. Let me tell you that a poorly designed double joint is worse than a well designed single joint.
However if you are big on do it yourself projects that are low risk something I have actually done before with Sculpey clay is to make another seperate joint part I could string into my single jointed doll using their existing joint and giving it double jointed function. In order to really get this idea to work flawlessly you would have to be very patient to get shapes and the fit correct. Even sculpting one joint gives a lot of respect for doll makers I think. still it's kind of fun, and there are definitely some possibilities to achieve what you might want, though ultimately, if you desire something highly refined and perfect, buying the body with joints you want is way easier.

*Notes, try not to get greasy Sculpey on your doll while you are working but otherwise oven fired Sculpey that is not super thin is durable enough to make joints and neck doughnuts and so on provided your doll is not super tightly strung. I never tried it in that capacity.

 

There is a tutorial on here somewhere that was about making the double jointed knee "peanut" out of apoxy. I think the biggest challenge will be making the channel for the elastic, but apoxy is extremely durable and can be drilled through with a dremel tool.