Have you tried combinable = true in whatever goods.ini you are using for them?

Exactly. That entry allows any item to be stackable. All commodities, ammo, bats and bots have this to true, all other stuff(guns, shields, thrusters etc.) to false by default. Though I am fearing that if you make a shield stackable that you might be able to mount multiple ones on the same hardpoint... maybe even without control how many you could mount, since they don't use cargo space there is no limitation as the commodities have...
And for multiple shield generators, you can still use the new program hardcmp to create HpShield02, HpShield03 etc. and define them in shiparch.ini. Been there, done that, works perfectly, and the number of hardpoints (interesting to mount them spread over a ship) determines the number of shields you can mount then.