No, you can't convert a movie to true 3D.
Here's what conversion is like
For the most part, they will go through and separate out elements into layers, and then create a new camera with those layers slightly offset to create the illusion of having the two 3D camera perspectives. That's why in some cases the elements in 3D conversions look like cardboard cutouts moving around.
In other cases, say if there's a closeup of a person or something is moving pretty far in distances they will actually recreate geometry and map the footage onto the geometry so it has actual 3D.
But there are various issues, like with characters it's difficult to deal with hair (since it's hard to separate out, and in some cases it's too complex to put onto geometry) or things like smoke or many particles (water splashes, dirt).
That's why 3D conversions have problems, there's a difficulty in figuring out actual 3D space, and then the issue of recreating background elements that would become visible to the new camera view.
For something like this, they would go back to the original 3D models that they used to create the shots as much as possible and the compositions where elements are already separated out (at least for FX shots). For instance, when they did Toy Story 1 & 2 in 3D since it was all computer animation they just went back to the original files and they already had perfect 3D.
So for Star Wars, many FX can get proper 3D since they already have geometry, but in any live-action or miniature elements they'll have to do the same conversion like I was explaining.