Yeah, this should be possible. I was planning on doing something similar but changed my mind on further reflection of gameplay. But anyways to answer your question, I think that you could take the preset ability the Vasari has for creating a siege turret.

Open this and then find the line that points towards the module file. Replace this with the module file you want. So if you want it to spawn a Phase Jump Inhibitor then just point it toward the appropriate .entity file. Let me know if this helps. If not, I will find the time to lay out a bit better instructions.

You may have to make the frigate FUNCTION AS a planet module... ...by copying over as much statistics that can transfer (and setting the acceleration and max speed to ZERO so it doesn't try to run off on ya)...

...unless of course you NEEDED the planet module framework for things like Tradeport functionality, etc... ...Then hopefully the DEVS will tell you the command, or that it's not possible...