Must one go into the xml files to do that?
Yes. Silverbeacher's reply #3 in the above referenced thread explains it best although I don't think it was mentioned at the time that you could get an extra planet by picking a normally PQ0 planet as your homeworld.
You can also do this same trick for the AI giving the galaxy an extra 9 planets that it would not otherwise have. Note that for some reason this doesn't work with the Krynn and Korath but that still leaves 10 combinations that will work. I actually at one point configured multiple AI's with the same homestar and homeplanet and it *seemed* to work even though that meant there were two stars both named Oxor in the galaxy, but given how long it takes to play a game I don't want to get to the end and discover some kind of problem and so I just take Korx and give the others out to my opponents. I usually leave out the Drath and the Terrans as opponents because they seem to have the best diplomacy and my strategy involves gaining an early diplomaic advantage so I give the Drath and Terran homestars to the Korath and Krynn.
Anyway the following are the useful possibilities for yourself and/or the AI.
<Homeworld>Korx II</Homeworld>
<Homestar>Oxor</Homestar>
PQ10, PQ8, PQ8
<Homeworld>Iconia II</Homeworld>
<Homestar>Icos</Homestar>
PQ10, PQ10, PQ4
<Homeworld>Toria II</Homeworld>
<Homestar>Torus</Homestar>
PQ10, PQ9, PQ6
<Homeworld>Thalia II</Homeworld>
<Homestar>Thalis</Homestar>
PQ10 (primary), PQ15 (secondary)
<Homeworld>Mercury</Homeworld>
<Homestar>Sol</Homestar>
PQ10, PQ10, PQ4
<Homeworld>Dratha V</Homeworld>
<Homestar>Drathis</Homestar>
PQ10, PQ10, PQ5
<Homeworld>Rathor</Homeworld>
<Homestar>Drengia</Homestar>
PQ10, PQ10, PQ5
<Homeworld>Epas</Homeworld>
<Homestar>Altaris</Homestar>
PQ10, PQ12, PQ3
<Homeworld>Arcea V</Homeworld>
<Homestar>Athol</Homestar>
PQ10, PQ10, PQ5
<Homeworld>New Iconia II</Homeworld>
<Homestar>Iconis</Homestar>
PQ10, PQ15, PQ3