Not quite sure what you are wanting, but anything not involving standard players pretty much has to be done outside of galaxy forge. Though its not too hard to get custom random militias if thats what you are wanting. Reguardless, you'll probably have to do some text editing on the galaxy file.

Alright, I take that as a you want to give the netrual malitia around a planet a boost. The easiest way to do this is to right click on the planet you want to add the forces to and click edit. Click add right below the group manager box and then add the ships/building you want to have around that planet. Now normally you'd then select what player you want to give them to, but you want to give it to a malitia. To do this, you'll have to save your map and then find the location of the folder all your map files (.galaxy) are located. Once you do that, find the file of the map you were working on and open it with notepad. You'll see the map in text format. Go to the planet you want to the malitia too and you should see the list of the ships/buildings you added in the forge. Now just change the owner to "RandomMilitia" and your all set. The ships will now be under the control of the malitia around that planet.

Note: Make sure you turn the default template option of that planet in the forge if you want only your custom malitias forces to spawn. Also, if it better suites your purposes, you can also change the owner to "Pirate" if you wanted pirates to be guarding an area instead.

Harpo,  i've done some tinkering with Goafan77's directions.  and in order to get the independent colonies like with the slider in the map designer, simply set the "colonize chance" field in the text editor to 1.  Also, if you are using templates, the planet owner and colonize chance fields for the template itself have to be set to "RandomMilitia" and "1" respectively.  and the owner is case sensitive.  just tried it on a map I've been working on, and it works beautifully, all i have to say is, I just gained to ability to put some serious roadblocks on the map.

p.s. if you can't find the fields, here's a sample piece of the code, i've highlighted the pertinent fields and you can find the specifications for the template(s) at the very bottom of the map's code:

planet
        designName "WeightedColonizable-3"
        inGameName ""
        type "TerranHome"
        pos [ 548 , 296 ]
        owner "RandomMilitia"
        isHomePlanet FALSE
        startingUpgradeLevelForPopulation 0
        startingUpgradeLevelForCivilianModules 0
        startingUpgradeLevelForTacticalModules 0
        startingUpgradeLevelForArtifacts 0
        planetItems
            templateName ""
            subTemplates 1
            template "claimed Colony"
            groups 1
            group
                condition
                    type "Always"
                    param ""
                owner "RandomMilitia"
                colonizeChance 1
                items 1
                item "Artifact:GalaxyTravel"
        spawnProbability 1.000000
        useDefaultTemplate FALSE

What until 7DS comes out, yup only 1.05 compatible but worth it.  I have heavily modified the GSDef with 4 racial militias, heavy patrols (don't actually roam ), and heavily colonized worlds of populated by many different races.

have tried the suggested code in a map of mine, but every time I tried to load sins(entrench) with the map in  a galaxy folder with the lines suggested had a CTD(crash to desktop) with empty log files,

im not sure if you noticed, but this thread is over a year old...

i noticed that the threat is over a year old, but i had the Problem and found the post over google. As i found my solution later for myself i posted it here to help others searching with google...

i will try the idea with the colonyfrigate 

At the moment i use the texteditor for that, its rather easy too...