What I actually wanted to get moving is the turret defense structures, not the starbases.
Is there any way to do that?
Yes, there is a way to do that.
basically you take a 'scout', ok? Then you save it as PlatformGun.entity or whatever name you want, just different than the original platform gun. I did this with the Advent Beam platforms.
You substitute all the icon and mesh and string calls with those from the original platform gun. You LEAVE the type and roletype of the scout as it is.
You will also have to change out the exhaust call with a blank, like this " "
Or leave the scout exhaust. It may look funny though, the exhaust coming out of the back of the platform with no engine casing or exhaust tube.
Then save it.
Then go to your Player entity file, and add it to the frigates call area. And add to the count.
Like frigate page 0
count 5
-add your new entity here, but remember to use the same format as the other lines.
Then change the count to
count 6
And save the file.
Now, since the scout has a certain speed, which you can see, on the thrust lines, there are about 4 or 5 of them,
you should probably tone it down from the scout speed, which is for the advent set at 220, to something like 100, or even 125 or 150 or so.
Then save again.
When you go to the frigate area in the menu, you will see the added ship, the platform icon (that you changed the line out of the scout entity with the line in the original platform entity) Also, if you want to get a bit more elaborate, you can edit the string file, by copying the original TWO lines, the name and the description of the name and function of the ship that is called in the entity file. PASTE it directly underneath and change the called name to something cool,
then change the CALLED NAME and CALLED DISCRIPTION in the string file, and change the CALLED NAME and the CALLED DISCRIPTION in the ship entity file to whatever, for simplicity's sake i usually call it frigate_whatevernametheshipis) 
Then add to the COUNT at the top of the string file, by 1 line. ( I know its two lines, but each STRINGINFO line counts as 1 and has both the name and the value, so its ONE line added for each of those STRINGINFO lines that you add.)
I know!
Its a lot to keep track of, but believe me, after you've done it a few dozen times, you can do it in your sleep. 
Anyway, your new ship in the Menu in the game will have your new cool name and the icon for the platform, which will make them easy to know which is which. And then look at the cost, if you want. Make the new platform ship something like 200 (original advent scout cost) or something else.
When you build them, you will now have a ship that LOOKS like the platform, has the guns of the platform (remember to change out the weapons call line, or adjust to what you want) BUT WILL MOVE, not only in the gravity well, but to other planets as well.
If you want something for the gravity well ONLY.
well, instead of building on top of a scout file (I did say copy it right?, so we don't lose your original scout file)
Just use a module builder entity file.
. The builders don't go out of the grav well. 
Anyway, that's it.

If you are really really really in a bind,
then tell me what faction you play, and what version of Sins, and I will make it for you. But you gotta try first, OK? Otherwise you never will get the hang of this modding thing by yourself.
-Teal