Accessories can't be automatically converted.
Within each accessory file there are a number of pictures of the accessory, showing it at different angles, for example: facing left at 90 degrees, facing left at 45 degrees, facing forward small (in the distance), facing forward large (close up to the front of the screen), etc. For the iD L, there are 4 pictures of each accessory. For the iD, there are 7 pictures of each accessory.
The accessory file also contains data that tells the Tamagotchi where the accessory should be located, relative to your character. This is so hats stay on top of you character's head, instead floating off or slipping down onto their nose. The file contains a block of data for each of the accessory's pictures. So, 4 blocks of this data for the iD L, and 7 for the iD. This data isn't overly complex, but it's would be very difficult to create from nothing. It would be necessary to try the accessory onto each of the raisable characters (with the character facing each possible direction she can face) in order to properly adjust the position of the accessory.