You don't need to convert from SVG to GeoJSON to use the image for a choropleth in d3, although it may be necessary for other libraries.
The path information on the SVG file (there must be at least one path for each state/country/state/subdivision in your map) is enough to map data to your SVG image in d3.
Let me put it in another way: if you can do a d3.select on your original svg that will return just one path per state/country/region then you can use d3 to build a choropleth. This is usually the case when you have a map in SVG. It is actually simpler than using a GeoJSON file as you don't need to worry about space location, projection etc. You may even want to tweak the original SVG to add missing information (like state names, that you can add as DOM IDs or Classes) before using it with D3 but that's usually not necessary.
On the other hand, as it was said before, SVG have just path/lines information, not geographic/spatial information, so if you want to convert from SVG to GeoJSON then you need to ADD/MAP geolocation information to your SVG image. For example, you can convert from svg to JSON first, then calculate/add geolocation information (this will depend on the region you are mapping) and generate a GeoJSON from that. As far as I know there is no out of the box tool to do this job for you though.