How Can I Include 4.2V, 8.4V, 12.6V, etc Power Supplies (e.g., Lipos)

It is in the .fzp file. This tutorial may help somewhat (although this is a bit out of scope):

to obtain the .fzp file unzip your fzpz file the fzp is one of the files zipped in there.

<module moduleId="hanmatek-HM305_2-CapacitorModuleID" referenceFile="hanmatek-HM305_2.fzp" fritzingVersion="1.01">

You likely need to add the CapacitorModuleID to the module id as that is what enables the code. Then the properties are in the properties section of the .fzp file (this is from the power supply .fzp file posted above)

  <properties>
    <property name="variant">variant 1</property>
    <property name="family">Hanmatek HM305</property>
    <property name="voltage">showInLabel=“yes” 9V</property>
    <property name="internal resistance">0.1Ω</property>
    <property name="layer"/>
  </properties>

the voltage and internal resistance properties are the ones of interest.

Peter