Here is how you can do it:
1) Download Nant.exe from http://sourceforge.net/projects/nant/files/ and check-in the bin directory which has Nant.exe.
2) Create a msbuild file (say msbuild.proj) with following code(change Path) and check-in the file.
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<Target Name="RunNant">
<Exec Command="bin\nant.exe; -buildfile:master.build build"/>
</Target>
</Project>
3) Go to Process tab in your build definition and then go to "Items to Build" --> "Projects to Build" --> Select(icon on the right) --> Add --> Change "Items of type" to "MSBuild project files" and then select the .proj file you checked-in.
4) Run your build and it should work. See below log that shows that it ran the Nant build file.
Build started 12/31/2013 4:55:20 AM.
Project "C:\a\src\F\Test Projects\TestProject\msbuild.proj" on node 1 (default targets).
RunNant:
bin\nant.exe; -buildfile:master.build build
NAnt 0.92 (Build 0.92.4543.0; release; 6/9/2012)
Copyright (C) 2001-2012 Gerry Shaw
http://nant.sourceforge.net
Buildfile: file:///C:/a/src/F/Test Projects/TestProject/master.build
Target framework: Microsoft .NET Framework 4.0
Target(s) specified: ; build
[solution] Starting solution build.