Is it possible to plan a descent in the FMC to reach an altitude XX miles before XXXXX?as example in the EGSS charts it says "Expect to cross 15nm before IDESI at FL200" ?

Sure, create a custom waypoint along the path and assign a constraint. See also: http://ops.precisionmanuals.com/wiki/FMC_WP_Entry_NotesPlease sign your full name in this forum.