Celestial casting requirements

[EternalSerpent]

Is there any spell or feat that would allow a non-outsider to count as an outsider for the purpose of casting outsider-only spells?

Thanks,

Eternal

[Shalvan]

Otherwordly, can be taken only at first level and I think it's not in PRC.
Some prestige classes can make you an outsider.

[xwarren]

Probably Aspect of Deity line of spells could do that, but we don't have it in PRC.
You can become outsider by using templates (fiendish, celestial, half-fiendish, half-celestial) or by taking some classes (monk, contemplative, acolyte of the skin).