I have a 200A main box out on a pole in the yard. I trenched and ran 4/0 aluminum to fee the subpanel on the house. Sadly I installed a smallish box and now I am out of breaker slots (too many 240's... ) Can I tap off the feed from the main box and run it to another subpanel or do I have to install another 200A (well maybe a on 100A) breaker in the existing sub panel to feed the seconds sub panel or (please say no) do I have to trench to the pole again and install the feed breaker there? If I can split the feed to both is there a special box made for the connection?

The easiest thing to do is replace the small panel board in the house with a standard 30/40 space panel board. The Homeline or Siemens panel boards from Home Depot should be fine.
The only real set back is that you should have run 2-4/0 , 1- 2/0, and at least a #4 copper or #2 alum ground from the outer service to the inner sub panel.
Your service is considered at the 1st main breaker location.
Hope this helps.

Yes, you can have a sub-panel feed from a sub-panel.
You can have a 100A sub-feed going to a new 100A sub-panel inside the house. NO reason to re-trench to the pole.