I think the ability slots are full so you would have to remove an existing ability.

Do you have the current reference files?  http://soase.weebly.com/modding-tools.html

How much modding experience do you have? Need to know where to start explaining.

Just so you know, the reference files with forge tools are out of date.

I have never done this but it should be possible with the communities help but I will get you started. Well I thought that building ships would have been an ability but its not.

All the file you should need are .entity inside the GameInfo folder. Look for StarBaseTech.entity. In that file you will see abilities and upgrades. These link to other ,entity files of that name.

UpgradeTypeCount 8
 UpgradeType "StarBaseUpgradeTechWeapons"
 UpgradeType "StarBaseUpgradeTechToughness"
 UpgradeType "StarBaseUpgradeTechHangar"
 UpgradeType "StarBaseUpgradeTechTradePort"
 UpgradeType "StarBaseUpgradeTechBlockColonize"
 UpgradeType "StarBaseUpgradeTechFactory"
 UpgradeType "StarBaseUpgradeTechResupply"
 UpgradeType "StarBaseUpgradeTechSelfDestruct"

 UpgradeType "StarBaseUpgradeTechFactory" is colose to what you want so open StarBaseUpgradeTechFactory.entity and take a look. This is where you pay for the upgrade.

You can also look at PLANETMODULE_TECHORBITALCAPITALSHIPFACTORY.entity. The only thing I see that would allow the building is the top line entityType "PlanetModuleShipFactory" so I am stumped.

Now we need someone with more experience if its even possible to do.

To my knowledge neither is possible.

Capital ships can only be built through capital ship factories. They can be spawned through abilities but this is not a viable technique as this method totally ignores capital ship supply.

I can't think of any modifier that would add labs to a starbase.

Besides the weebly look at the link below for reference information related to moddable files. There was an attempt at a general modding wiki but last I remember it didn't get very far.

http://code.google.com/p/soaseplugin/wiki/Syntax

BEtter off turning your capship factory into a baddass weapons platform with repair ability and maybe anti matter refill station. 

Take a look at the Guass Defence and how it upgrades and uses different mesh (model) when upgraded. You can make a copy of the CapShip factory and make 2 kinds. 1 with more research. You can use the starbase mesh for that.