Yes, but I have been reading here and there is a lot of talk about most or all mods pre-Crusade being broken...

There are several files that impact planet number and class

A quick and simple way to increase planet class is to mod (must be a mod not a change to core came file) GalCiv3MapDefs.xml

e.g.
    <DefaultPlanetClassSelect>
      <MinPlanetClass>6</MinPlanetClass>   increase
      <MaxPlanetClass>10</MaxPlanetClass>  increase
      <Weight>15</Weight>      % distribution

There are several sections ans the total <Weight> must - 100.  So another way is to decrease the weight of the smaller class entries and increase the larger ones

hope that helps

PlanetTraitDefs.xml

change the min and max for planets like so

  <PlanetTrait>

    <InternalName>DesertWorld</InternalName>

    <DisplayName>DesertWorld_Name</DisplayName>

    <Description>DesertWorld_Dec</Description>

    <RelativeWeight>10</RelativeWeight>

    <PlanetClassSelect>

      <MinPlanetClass>50</MinPlanetClass>

      <MaxPlanetClass>60</MaxPlanetClass>

      <Weight>100</Weight>

    </PlanetClassSelect>