The most reasonable thing I can think of is to say that a Shadow spell can emulate any spell the caster is aware of/has experienced.
It's assumed (though I suppose this isn't WRITTEN anywhere) that an arcane caster is aware of all the spells in the PHB, as well as any non-core spells that the game permits in the setting (that aren't otherwise kept secret).
So if, as DM, I say in the setting doc, "You can use any spells from PHB, T&B, and BOEM," then it's fair to say all those spells are known by any arcane caster with Knowledge (Arcana). They're public domain spells.
By that logic, if a character researches a spell that obeys the strictures for a Shadow spell, he can be said to know it, and thus to adapt the Shadow spell to emulate it.
Similarly, I'd say if a character can study a custom spell (perhaps in a captured spellbook), or has experienced it in combat, he could also then pattern a Shadow spell to emulate it...if it fit into the appropriate guidelines.
A mean GM might require the character to fail his save though...and thus experience that spell at its absolute worst magnitude. If you make your save you're not REALLY experiencing the full monty, after all.
