As white against the Sicilian Scheveningen (1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6)  I, like everyone else, enjoy playing the Keres attack (6. g4) to strike fear into the hearts of my opponents and watch them cry as they realize that they no longer have a chance at avoiding a loss.  

Well, proponents of the Scheveningen got tired of losing on move six every time, so the started playing the Najdorf (1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6) through move 5, with the intention of playing 6... e6, basically transposing into the Scheveningen while making white play a sixth move that isn't g4, and making playing the Keres attack impossible.  

So, my question is (finally), as a player that plays the Fianchetto variation against the Najdorf (6. g3), if that Najdorf becomes a surprise Scheveningen, can I play  7. g4 and enter into the Keres attack, or is that extra tempo gained by black enough to render the Keres attack useless?  Is it more advantageous to just stick with the fianchetto lines?

I don't think the Keres is strong to the point that you would sacrifice a tempo for it.

well, the idea though is that they don't just get a tempo, they specifically play a6, a move that doesn't seem quite relevant to the king side attack in the Keres.  

I agree with DritzzT.

How about playing the English Attack (6.Be3) against the Najdorf and then if Black decides to follow up with 6...e6 (insted of the Najdorf 6...e5) transpose into the Schevenigen play 7. g4 in 1 move? Be3 is played often in the Keres Attack anyways.

Imo, if you want to play the Keres, you should also play the English Attack (Najdorf) as well as the Yugoslav (against the Dragon) since they all have similar styles of attacking compared to the relatively quieter 6. g3 against the Najdorf. 

By playing g2-g4 in 2 moves, you have allowed black some time to consolidate his position and especially in openings that involve a lot of attacking (aka the Dragon or even the ...d6 Sicilians in general), one tempo loss can be deadly. It might not be that "deadly" in this case but remember that in  the Keres, white usually emerges with a +/=. This tempo loss would probably drop it to just =

The extra move black has (a6) is actually much more useful than it looks and actually allows black to equalize on the spot:

But don't loose all hope, you can still get into a type of keres attack by first playing 6. Be3! (English attack) then if black goes 6...e6 you can follow up with 7. g4!? which is very similar to the keres attack (but it should be mentioned that it isn't as strong as the real one but still very playable and dangerous for black). And since you seem to like these aggressive lines you will also get the type of play you want if black goes for another move like 6...e5 instead.

Maybe I should also demonstrate why 7...d5 doesn't work as well for black when white has a bishop on e3: