Entropy gathering from already generated bitcoin keys? Say someone has generated n bitcoins. Now, these are from very good entropy. When regenerating new keys, would it be a good idea to take the keys for entropy improvement? On the one hand, the entropy is already destinated on the other hand, it comes from the best sources? As special case, would talking only parts of it be a better idea?
In basic terms, entropy is good when it is secret and not good when it is known to the attacker or guessable.

Adding the keys would not improve your already existing entropy (although they wouldn't worsen it either). They are just derived from that entropy, so an attacker who breaks your "very good entropy" can just compute those keys. He can also just compute the required parts.

If you have "very good entropy", just use that for how ever many keys you want. Any decent PRNG should have a sufficiently long period until it starts repeating.