If you are doing a pre-sequence, you can put anything you want in the sequence. And, if the result is not exactly what you want, you can always go back into the sequence and edit any note to move it where you want. You can also get the results you want in the “Pianist” mode by playing a low C and sustain through the Dm chord. I assume, though, that you want to use the fingered mode when you play with the chord pre-sequence.

I believe you can force a C in the bass line while you are in the fingered mode by turning on “ON BASS” and using a Dm7 chord with the C at the bottom of the chord. I can’t really hear that much difference in the results when I simply play a Dm7 chord when I see a Dm/C chord although that normally results in a D in the bass line. You have to be careful with the inversion used because it can result in a F6 result with some inversions. Arranger keyboards do a fantastic job of interpreting what we play, but some things are just not possible to interpret because different fingerings of the same keys result in different chords depending on the inversion used.