I'm not expert on music, but my research shows that a Cadd9 chord differs from a C9 chord in that it doesn't include the 7th note of the scale, but the 9th note. So a Cadd9 should include the notes C-E-G-D.
Essentially, it's just a C chord with the 9th note in the scale added. Now obviously, there isn't a 9th note, so we loop back into it and in the C-scale that would be a D.
Essentially, you just finger a C-chord on your banjo and lift up your middle finger on that 4th string. Now your 4th string is an open D, which would give you a Cadd9.
At least that seems to me how you could finger it.
But in my research I saw where some fingered it this way:
4th D String: Open
3rd G String: Pinky finger on the 5th fret
2nd B String: Middle finger on the 3rd fret
1st D String: Index finger on the 2nd fret
Same notes, just a different fingering. Try em both out and see if they sound the same as the guitar chord you're talking about.
I could be totally in left field here. If so, somebody smarter than me can chime in. Or maybe someone can chime in if my understanding of how to finger a Cadd9 is wrong while the 2nd one I mentioned is right.