Cece, let's get back to basics.

When you say you are trying to transcribe from piano to accordion, do you mean you have the sheet music for piano? If so, what key is the piece in? Look at the beginning of it. How many sharps (tic-tac-toe grids) or flat sign (funny-looking b's) are there. If neither, the key is in the C family.

If you don't have the music and are sounding it out, how many black keys do you hit when playing most of it?

If F's are sharp, then D F# A C is the D7 chord, for which the accordion has a button.

IF F's are natural, then try a Dm button and let the C note which is almost certainly in the melody take care of the C.

If neither of those sounds good, simply hit an A note a couple times, because A goes with D,F,F#,A, and C. There is a button which will play just an A.

Most of the time when you have these funny chords, they only last a little while, and who's going to notice? However, if you feel this measure is key to the mood of the piece, find a friend who plays accordion to help you out.