For the verb こもる, the thing that is doing the 'filling' is usually followed by a が, like in the example you gave. That is just how this verb operates. You can see more examples [here][1] if you like.

If you want to talk about someone actively doing the "filling", you can use the verb こめる (込める), where the thing to be "filled" is followed by を:

> 僕は気持ちを込めて歌を作曲したいです。

> Natural translation: I want to make songs with feeling.

> Literal translation: I want to make songs and put feeling into them.

About your question regarding "Tanaka is filled with happiness", I think your Japanese translation isn't natural because of how こもる is used, and think the following would be a bit more natural:

> 田中さんは幸せでいっぱいです。

  [1]: http://dictionary.goo.ne.jp/je/27239/meaning/m0u/%E3%81%93%E3%82%82%E3%82%8B/