hi Mags.
currently the only 'hardware' way to do what you're asking, is to use the Pioneer Toraiz Sp16 sampler which will sync to the BPM of the CDJ/XDJ's via the ProDj link and also sends out midi so you can hook up any drum machines, synths etc and they'll follow any tempo/pitch changes on the CDJs.
CDJs do not give out, receive or sync to any kind of midi clock, they use Pioneer's own ProDJ link format over ethernet.