I found this thread for TeamCity 7.x:
In that thread the user reported using the TeamCity NUnit launcher to specify parameters to pass to NUnit, including the location of addins. I don't see an NUnit Launcher step available in 8.x, only NUnit, and the NUnit step does not give the option of specifying an addin.
(I'd also like to figure out how to specify the same addin for ReSharper when it runs my NUnit tests)
Found out where to put it in ReSharper. The ReSharper->Options->Unit Test->NUnit tab has the information on that one. Now just have the primary issue of where to put it in TeamCity.