Hello Abdelaal;
Yes, to connect any PA instrument to a DP network you need a network interface module, either a DP/PA coupler or a DP/PA Link.
-A coupler allows you to connect the PA instrument as a DP slave, through its gsd file; all the DP network must operate at PA speeds (45.45 kB, if I remember correctly).
Here is an extract from a FAQ on this subject:

Direct connection of the DP/PA coupler to the PROFIBUS DP:
The DP/PA coupler is not configured explicitly in the HW Config. You configure your PA devices like DP slaves. Addressing is direct from the programmable controller.
The bus addresses of the PA slaves after the DP/PA coupler must not overlap with the those of the nodes of the associated DP master system, because these field devices are considered as being connected to the DP bus with regard to the addressing.
http://support.automation.siemens.com/WW/view/en/21562130

-A Link (IM153-2 + 1 or more couplers) allows the DP network to operate "independantly" of the PA, at its own bus speeds, up to 12 MB; the PA side operates at its own speed limitation. The link is seen by the Profibus configurator as one single Profibus node; but you must build a specific gsd file that will include all instruments connected top this node. Siemens has a downloadable tool for this (gsdtool), java-based, that integrates all gsd files for each instrument and allows you to configure the PA side of the link. A little more work and a slight learning curve, but worth it if you need to maintain the speed of the DP bus.

Hope this helps,
Daniel Chartier