In the MP3 encoding settings:
First, select "CBR/ABR Encoding", "Target Bitrate: 64 kbps", and check "Restrict encoder to constant bitrate (CBR encoding)"
Then, select "Custom Command Line Switches (advanced)" and add --resample 44.1 to the string.
The complete command line is lame.exe --preset cbr 64 --resample 44.1
This will probably lower the audio quality even more than the default 64 kbps CBR setting would do, but you will get what you want.