The bit where your home planet is created is this bit:
-- Asteroid 11
-- gravity variables
roid = 11
AccelerationX[roid] = 0
AccelerationY[roid] = 0
MomentumX[roid] = 0
MomentumY[roid] = 0
density[roid] = 0.75
CoordX[roid] = CoordX[0] + 23500
CoordY[roid] = CoordY[0] + 20250
roidradius[roid] = 1075
-- Creation
a = AddAsteroidWithAttribs(CoordX[roid],CoordY[roid],(math.random(45,90) / 100),(math.random(45,85) / 100),(math.random(30,70) / 100))
a.Owner = 1
a.TreeCap = 4
a:SetRadius(roidradius[roid])
a:Reveal(1)
a.Moveable = False
s = a:AddDysonTree()
s = a:AddDysonTree()
s = a:AddDysonTree()
a:AddSeedlings(62,1,0.25,0.01,0.01)
Lines 363 - 385.
"a.Owner = 1" sets the asteroid to be owned by Empire 1 (the player).
"a:AddSeedlings(62,1,0.25,0.01,0.01)" gives the player 62 seedlings with the specified strength, energy and speed.
If you wanted a different asteroid, just change one of the others to be yours, and add some seedlings on it.
You might also need to use the Reveal command, otherwise it will end up being the sort of friendly asteroid you "discover" later on.