Q:

Origin of 楽しみ and 楽しみにする

楽しみ is derived from  楽しむ isn't it? Despite this, it's used as if it were an adjective, and I don't completely understand why it can be used in 楽しみにする.
What is the name of the form that 楽しみ is relative to 楽しむ, and why can you use it in にする?

A:

Another set with the same conjugations is 悲{かな}しい・悲しむ・悲しみ　(adjective/verb/noun), where 悲しみ means, more or less, 悲しむこと.  In some cases, this noun form of a verb has picked up additional meanings along the way.
楽しみ is something of a special case in that it has at some point acquired both the status of a na-adjective, and a meaning of anticipating that some future event will be 楽しい.  Hence, you can say 楽しみに待つ (but probably not 悲しみに待つ).
That said, AをBにする, where both A and B are nouns (and the Aを part may be dropped) is a common grammar pattern with the general idea that A is made to be/treated as/viewed as B.

