First, you should know about the Strothers String Gauge Calculator , which will calculate an appropriate string gauge once you enter the vibrating string length (VSL, or the distance between the bridge and nut) and the note you want to play. The calculator errs on the light side, so feel free to go one or two gauges heaver.
Second, can I ask how you play and why you want to tune this way?
If you play in a drone style, ADa is considered a "reverse ionian" tuning, meaning you would still be playing in the key of D, but the drones are reversed, with the root being on the middle string and the fifth on the bass string.
If you play chords and fret all strings, ADa is a common tuning for baritone dulcimers when the player wants to play in the key of D to play with standard dulcimers tuned DAd or DAA. In the case of baritone dulcimers, the middle string would be tuned to the same D as the bass string of a standard dulcimer, with the bass string a fourth below that and the melody string a fifth above.
The 3/4-size instruments such as the McSpadden Ginger or Ron Ewing's baritone dulcimette are sometimes tuned to A as well. They would be an octave above the baritone, with the bass string being the same as the middle string on a standard dulcimer.
Unless you are using the "reverse ionian" tuning to play in D, however, a normal tuning in A for either the baritone dulcimers or the octave versions of the baritone dulcimers would be AEE or AEa.
It sounds like you are trying to get the Ginger tonal range on a standard dulcimer. You can possibly do it, but you will need to identify the correct string gauges sing the calculator linked to above. But note that most people who tune that way use a smaller dulcimer, not a standard-sized dulcimer.
And I'm still curious why you want to tune this way.
Dusty T., Northern California
As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie
updated by @dusty: 12/02/19 07:01:34PM