Hello M. le meusien;

If you want to interconnect 2 S7-400 racks, (a central rack and an expansion rack) you need to use Interface Modules. You can have a look at the attachment, it shows the introduction to the chapter on IMs from the S7-400 Modulespecifications manual:
http://support.automation.siemens.com/WW/view/en/1117740

If you wish to use Profibus-DP to connect to remote I/O modules, then you cannot use a S7-400 expansion rack as a slave. You can use any SiemensET 200 famly unit (ET 200M, ET 200S...) or Wago Profibus-DP units, or... The manufacturer's list is very extensive.

Hope this helps,
Daniel Chartier