The listing merger tool was available when the variation format was very first introduced and only available in the fashion category - it didn't really work, though, and the tool was retired pretty much as quickly as it was introduced.
Unfortunately, the only thing you can do is end all the listings then create a new variation listing for them - I had to do this with quite a few items. I tended to do it over several weeks, so my popular listings didn't lose traction all at once. I actually would create the variation listing first, then edit the listings I was going to end to let people know the items were still available (i.e. I changed the images to a graphic saying the item was relisted with more options, and the description was just a link to the new listing).
