I tried to register a Python executable installed in a virtualenv with RegisterExternalEvaluator but it didn't seem to work:

In[162]:= virtualenvPython="/Users/meng/python_virtualenvs/virtualenv-test/bin/python2.7";
RunProcess[{virtualenvPython,"--version"},All]
Out[162]= <|"ExitCode" -> 0, "StandardOutput" -> "", 
 "StandardError" -> "Python 2.7.13
  "|>
In[161]:= RegisterExternalEvaluator["Python",virtualenvPython]
During evaluation of In[161]:= RegisterExternalEvaluator::invalid: -- Message text not found -- (ExternalEvaluate`Private`reason)
Out[161]= $Failed

For those, who like me, don't want to modify their system python (even by installing virtualenv), here's a hack to get things working:

First, make sure that you have successfully registered an external evaluator for Python. FindExternalEvaluators[] should return it.

Then, we trigger loading the ExternalEvaluate framework (in case it hasn't been loaded yet):

In[1]:= ExternalEvaluate
Out[1]= ExternalEvaluate

We manually start up a session with a string return type, and assign it to appropriate the internal variable

In[2]:= ExternalEvaluate`ImportExport`Private`$ImporterPythonSession = StartExternalSession["Python" -> "String"]
Out[2]= ExternalSessionObject["9a23363e-5403-442d-8fbd-39f9aff7dc48"]

And now it works!

In[3]:= py = StartExternalSession["Python"]
Out[3]= ExternalSessionObject["5022a77c-eedb-4d39-bbb3-c8b6a644ecdf"]

In[4]:= ExternalEvaluate[py, "1+1"]
Out[4]= 2

Note: I posted this solution on the Mathematica StackExchange, where it is more likely to stay maintained than here (say, if a paclet update or 11.2.1 fixes this problem).