As a die-hard d20 fan let me say this, Mage is probably the only game I could think of that d20 just wouldn't work for (although I'd still rather play WoD in the original system, I could cobble together a conversion for the other games). The sphere system is so open-ended that it's almost antithetical to the highly regimented, highly documented, highly controled nature of d20. Magical progression is tied strongly to roleplay (seekings) and without good roleplaying, no amount of XP will advance you.
If you really wanted, you probably could just bolt the sphere system onto d20 Modern and call it Mage (if you must use d20, use d20 Modern since it's actually written for a modern-day game, and there's no need to invent a library of modern classes). The problem is, magical progression happens to every mage without regard for what else they do in life. No matter the class, every mage learns magic. A martial artist or a raver could be a much stronger mage than someone wearing robes and a pointy hat or ecclesiastical robes and clutching a crucifix. So magic and class don't really go together.
If you really want to play mage, you're best off learning the Storyteller System, it's not that hard and it really does express the setting best.