I have a Siemens 100-amp main panel (Catalog #: S2020B1100), and it only has
one neutral/ground bar. The neutral/ground bar is on the right side of the
panel.

I am trying to figure out if I can add another neutral/ground bar on the
left side and, if so, how to do that.

The information about Siemens service panels is located at:
http://www.hqs.sbt.siemens.com/gip/g..._App_Guide.pdf .

This panel is a Siemens "ES" Series Load Center, and it has 20 circuits. On
page 3 of the above online guide it says, "Single sided neutral on 24
circuits and below". So that explains why the panel that I have only has
one neutral bar installed.

My question again is, can I add another neutral/ground bar on the left side
and, if so, how do I do that? Thanks.