A question about index internals

Hi,

I've been working with SQL Server for a long time and many times I had to
change how an index page is filled with fillfactor clause. I've noticed that
PostgreSQL doesn't have anything like that, am I right?

I'd like to implement something that could change the index page filling. I've
found a struct called IndexOptInfo. Do I have to change it? Also, I followed
the creation index path and I reached a fuction called "heap_create". Am I in a
right way, or completely lost?

Best regards

Eduardo Morelli