CONTAM is not part of E+, but you can use it with E+. As for whether you need to, that depends on what you want to do. In any case, there are a couple of different paths for multizone airflow calculations with E+:

• There's AirflowNetwork, which is built-in to E+. It is not supported in OpenStudio right now, though I am working on fixing that. Check the EnergyPlus documentation (both the engineering and I/O references for more information).
• OpenStudio can export ContamX 3.1 input files. This is what osm2prj was doing (assuming you're talking about this osm2prj: link). That's code I wrote a long time ago and it is behind the current version of OpenStudio. I'll eventually update it, but using it requires you to build OpenStudio. So the preferred route is via the Ruby bindings to OpenStudio. Once I verify that it works, I'll be posting a Ruby script in that GitHub repo that does pretty much what the C++ does. Be aware that the translator creates a model that makes some assumptions about the OpenStudio model. The new Ruby script includes documentation.
• CONTAM 3.2 (the current version) includes more cosimulation support, you generate your model using ContamW and then you can cosimulate. Well, that's what I understand, as I haven't personally had time to try this out. It's on my list of things to do. The documentation is here, there's a section entitled "Working with EnergyPlus" under "Using CONTAM".

There are a couple of other (commercial) options, but I don't have experience with those.

Update: The first version of the Ruby script is available here. It should work for OpenStudio version 1.11.1 or greater.