Interestingly enough, Savage species actually does indeed allow you to do it via the rituals listed Chapter 11 - Becoming a Monster. They even address this very case and purpose:
If you are a DM, chances are your players will want to
build new characters when they see this book. Some,
however, will want to transform their existing characters
into monsters. This chapter provides information to use
when that happens.
Some characters may wish to gain the benefits of a particular
type or subtype of monster, without actually becoming a
specific kind of monster. For instance, a character may wish
to gain the goblinoid subtypea character may wish
to gain the goblinoid subtype in order to use goblinoid specific
magic items. Another character may want a specific
subtypemay want a specific
subtype in order to qualify for a prestige class. Minor rituals
allow these sorts of transformations. Minor rituals
allow these sorts of transformations.
If you are looking for more impact in the change, a major ritual will allow you to take on a monster class through LA or class levels at the cost of the levelsexperience points you already have, SS Pg. 149.