If you configure it as a six string with extra bass, you can tune the two lower strings as you like or what suits the music you're plying. Generally speaking the 7th and 8th are D and C respectively but D and F# (a kind of re-entrant tuning where the F# is 6th string 2nd fret pitch equivalent) is also very handy. Or if you play jazz one of the two is handy as an A and the other a B or B flat.
Most often for the configuration of one extra bass and one extra treble, the low D as you mention and a high A works well. That keeps the ratios between the strings familiar and any higher the available strings get pretty thin sounding. Classical guitars as they are built don't respond well to thin strings.
Those are just the standard/most common tunings, there are others such as the Russian 7 string with an added bass, D G B d g b d' and as sometimes a low C is required in the music so you could tune the 8th to C....or find your own tuning that works for you.
Edit. Oh sorry Guitareleven, I posted at the same time so missed your post. Didn't mean to repeat your answers.
Scot Tremblay Guitars
"One picture is worth a thousand words. So, for me, one good note put where it should be put, will say what it will take some people many notes to say. ~B.B. King, 1986