Master of Orion/ModdingHelp
A random star map is always generated at the start of the game. However, edited savegames can be (ab)used to create custom designed scenarios.
The game whose save is to be used should be started with the desired difficulty level, galaxy size and number of opponents.
Of the currently listed save game editors, none is a complete solution for scenario design as they were primarily developed as cheat utilities. MapEd can move the stars around, RaceEd allows selecting which races are in play and OREO allows some in-depth planet editing including star color.