Now, this is pretty simple, and sholdn't take you long to master, so I'll get this over with quick. Don't forget this only works for Galactic Conquest maps.
Open Planets.xml, in your data->xml folder (use the UEAW beta, testers.)
Here's the code we'll be editing (I used Alderaan for simplicity, but you could use any planet that has a similar climate to the one you're adding):
<Planet Name="Alderaan">
...
<Galactic_Model_Name>W_planet_Asteroids.alo</Galactic_Model_Name>
...
<GUI_Model_Name>W_planet_Asteroids.alo</GUI_Model_Name>
...
<Special_Structures_Land>0</Special_Structures_Land>
...
<Planet_Surface_Accessible> No </Planet_Surface_Accessible>
...
<Land_Tactical_Map />
<Space_Tactical_Map>_Space_Planet_Alderaan_01.ted</Space_Tactical_Map>
...Right. I'll be showing you how to change Alderaan to Sluis Van, mainly for testing purposes. To add an extra planet, you have to edit more xmls and other stuff, which I won't go into here, but maybe later

Change the Planet name to Sluis Van:
<Planet Name="Sluis_Van">Change the Galactic model name and GUI model name(you won't have to do this if you use a planet that isn't already destroyed):
<Galactic_Model_Name>W_planet_Urban.alo</Galactic_Model_Name>
...
<GUI_Model_Name>W_planet_Urban.alo</GUI_Model_Name>Change Special Structures Land(to whatever number you like between 1 and

:
<Special_Structures_Land>6</Special_Structures_Land>This is the important bit, and all you really need to do if you're feeling lazy:
<Land_Tactical_Map>Sluis_Van_Land.ted</Land_Tactical_Map>
<Space_Tactical_Map>Sluis_Van_Space.ted</Space_Tactical_Map>Obviously you name them for the maps you're using/testing.
And that's about it. Hope that helps map testers!