You can try to change the tuning, if you need certain "missing" notes. However, that is limited by the string tension. This table is still helpful for transposing songs: If you figure out which key the song is in, you can add the same amount of half steps to all notes until you are able to play it on the D and A strings. If you have sheet music: Look at the key signatures next to the treble clef, count them, look in this table to see which key the song is in.
If you have the notes of your favorite song and you want to play it on a short Dulcimer Guitar: First look at the lowest and highest notes you'll need. Because if you need a E3 and a G5, well, you're out of luck.
And if you can't read sheet music or - even worse - Solfege: I added a handy (and ugly) Cheat Sheet underneath.
A comparison of different Fretboards. On top the diatonic (stick) Dulcimer board up to the 9th fret. Mountain Dulcimers are similar, but with more frets repeated after the 9th and tuned "upside down". A chinese Erhu has very similar tuning (D, A) and finger positions to the Dulci. In the middle is a basic Saz/Baglama fretboard. There are a lot of variants with the tuning, the order of the tuning and the movable frets. Sometimes there are even additional half-quarter frets and sometimes the frets are slightly off center (e.g. to get #2 b3 instead of the #3 b2). At the bottom is a default Guitar with it's chromatic half-step-fretboard.
The fret lines are not at their "natural" position, they are normalized to clearly display the half and whole note distributions. For example the first E fret on the Dulci is a lot closer to the nut then that fret on the Saz, because the Saz' neck is a lot longer. Also: The way how frets get smaller along the fret board is also not displayed here.
Diatonic Stick Dulcimer with the bonus fret 6 marked with a triangle. And the octave fret 8 marked with a line.
And a long-necked Saz/Baglama/Kopuz with frets (perde) marked analogue to the Dulci. Top number = Saz fret, middle = Dulci fret number of the same value, low = Guitar fret of the same value. Fret 17 is the octave and marked thicker. Compare this to fret N° 8 of this Stulci and note how the frets are stretched almost twice as long on the Saz.