Thank you all for your comments.
I figured out what I was doing wrong (operator error...). I was not selecting the format to send, I just clicked "OK", which assumed I wanted to send an epub format (the first in the list), necessitating a conversion of those books that did not have epub formats available. I was misinterpreting the message "No suitable formats" to mean that the formats I selected were not compatible with the device. The message actually means that the formats available for the books that I wanted to send did not include the format selected in the popup.
This, however, presents another problem. The popup only allows the selection of one format. If I have selected several books of different formats to send, I can only choose one format in the popup. Books not of this format will either be converted or not sent, depending on how I respond to the autoconversion notification popup.
I really think that declining the autoconversion should send the books unconverted.