Hi Chuck,
Assuming 50 ohm, and the Hz in dBm/Hz and nV/sqrt(Hz) is 1Hz, you can simply perform the conversion from dBm to nV.
nV/sqrt(Hz) = 10 ^ ((dBm/Hz + 166.9897) / 20.0);
If you have access to Matlab/Octave, an anonymous function can make quick conversions
>> myconversion = @(dbm) 10^((dbm+166.9897) / 20.0);
then later
>> myconversion(-160)
ans = 2.2361 Regards,
Andrew