its just like putting a magnifying glass in front of your lens....nothing to do with the focus motor...these type of convertor can convert ur lens wider or convert it to Macro lens, or telephoto...its cheaper to buy a convertor than to getting a new lens...
in fact..not neccessary to get Nikon brand...there are 3rd party converter around...like raynox....
can check out this site
http://www.mcgill.com.sg/shop/default.php?cPath=58
but than again..there are hard core fan die die swear by cannot use any 3rd party things on their cam...no offense to any of these fan..I am just providing another options...
but pls take note that a convertor will reduce your IQ....cos ur lens have to go thru another layer of glass...but to me..the pic looks nice to me..I am not a pixel peeper...