Say if I add Extra Pale malt, then add some wheat, oats, or something else to add body/weight only to the beer, can it still be called a SMaSH?
I would think not. SMaSH means single malt and a single hops. The point is simplicity and purity, where emphasize the flavor of a single malt and single hops. Adding additional ingredients will change the attributes of the beer, which will means that it is no longer a single malt.
But IMO, the more important question is why do you care? Beer is a beverage that should be enjoyed. If you feel that adding additional grains will give you a more enjoyable beer, do it. In fact, one of the main reasons why people homebrew in the first place is to experiment with different flavors and to create a beer that can me more enjoyable than what is offered as standard, so making a SMaSH beer which isn't exactly SMaSH makes it YOUR beer.
According to Drew Beecham's article in November/December 2012 Zymurgy and associated presentation (subscription required), the term
SMaSH was coined on homebrewtalk.com and, of course means,
Your question highlights part of the problem with SMaSH recipes...they can be boring or lacking a quality you desire.
So although adding an additional grain to your recipe would disqualify it as a SMaSH recipe, Drew proposed
Brewing On The Ones, which allows for a single malt plus a single color or flavor malt or adjunct. Still one hop, but additionally, one spice, one sugar.
Drew's rule (for Brewing on the Ones): You get one choice per "category". That means one base, one adjunct, one hop, one yeast and one surprise.
Generally a SMASH is a simple recipe to experiment with a single ingredient at a time
To be true to the acronym one would only be able to play with base malts and bittering hops. But this isn't the case.
IMO it was never intended to be strict to the acronyms meaning. Single malt, single hop.
But rather single malt, single hop aside from a well known base malt and bittering hop or better yet hop extract for bittering.
So to explore how ingredients effect and add to the simplest recipe.
SMASH base beer.
X lb 2-Row.
X ml hop extract at 60 min.
One then could add their specialty malt OR hop addition to explore the effects.