No reason why that I can think of. Use the 6th, 5th and 4th strings from any six string set - those are 'standardly' tuned EAD and work great for DAD, especially with a longer scale like you have. Musicians Friend has a brand called Musicans Gear that is cheap and good.
Or look down lower right on the main page and see info on strings for different tunings.
I don't know if this helps, but here is a link for electric D Major tuning
Good info... Thanks for the help.
using a DAD tuning on my git. its a good tuning..i have a shorter scale 600mm ( 23.62") . i used the 3rd,4th and 5th strings from a set of cheap electric guitar light set of 9's
Best way to choose string gauges:
String Diameter Calculator
I made a pair of canjos with this scale length and tuning, but used completely different strings for them. one had EAD strings with the E tuned down and the other used the Gbe Strings tuned down. I'd recommend trying the Gbe strings, gives off a good jangled chime.
got the idea from an alternative DADDAD tuning used in Scottish and Irish Folk, just a matter of splitting up a regular 6 string set. plus you still got the EAD for another DAD if you wish.