RandomPrecision said:
It's always been my impression that you can add the tiefling properties to a humanoid creature. There's a 'Tieflings as characters' section in the Monster Manual that allows it as a playable race with an ECL of +1.
No, by the RAW, they're a distinct race, not a template, so you can't take an elf and add the tiefling template, because there is none, at least not in the core rules.
(perhaps there's something in FR).
No tieflling/drow in the FR, though there is a special kind of half-fiend, the Draegloth.
All that said, I think it would be appropriate to extrapolate a tiefling template (as well as aasimar and genasi) from the respective write-ups.
Which would be like this:
Tiefling (Inherited Template for humanoids)
Type: Change to Outsider (Native, augmented humanoid)
+2 Dexterity, +2 Intelligence, -2 Charisma.
Add Darkvision 60, or increase by 60 ft.
+2 racial bonus on Bluff and Hide checks.
Special Attacks (see above): Darkness.
Special Qualities (see above): Resistance to cold 5, electricity 5, and fire 5.
Level adjustment +1.
That said, there are several types of variant tieflings out there, which are the result of coupling between a certain kind of humanoid and a fiend (often, the kind of fiend is specified, but not always).
There are Durzagon (Duargar/Devil as far as I can remember)
Tanarukk (Demon and Orc)
Fey'ri (Demon - strongly suggested Succubus - and Sun Elf)
And probably several others I cannot remember at the time.
Those types of tieflings aren't just a combination of the racial package and the tiefling stats. Fey'ri, for example, get to choose demonic abilities out of a list (all of which are lessened versions of Succubus powers) and can change their shape at will, into any humanoid shape.
This, of course, is maybe an even better method: Instead of taking a drow and slapping the adjustments extrapolated from the tiefling entry on it, you create a distinct drow-tiefling.
Maybe something like this
Planetouched: Tiefling - Drilith. These special kind of tiefling is the result of a ritual coupling of Drow and Marilith, a ritual that has recently been invented and used by the followers of Selvetarm. They get the usual drow stuff, and in addition (or instead of the normal stuff, they get +2 Int, +4 Dex, +2 Cha, 4 Arms, +4 listen and spot, 1/day see invisible, alter self, dimension door, mage hand and protection from good. Maybe give them some racial hit dice, perhaps as much as 4, and an appropriate level adjustment.