I can offer a workaround of sorts.
When adding a custom race in the custom race screen you can name their homeworld whatever you like. Choose one of the existing homeworlds and name it as you wish.
In combination with this add a new ability entry to the AbilityDef.xml with the following two triggers in it.
The first trigger will add an additional world to your races starting system, you can add it to the ability as many times as you wish as set the two <ValueParam> entries as you see fit to generate as many planets as you want. These planets will show up on your second turn this is important because if they show up on the first turn they will be affected by the second trigger and it will throw off their planetary values.
The second trigger will add tiles to your starting homeworld so you can set it up to be whatever quality you desire.
As far as adding any specific planetary traits that is not something I know how to fudge at the moment.
Hope this helps.