I wouldn’t try to reuse another Dataset if your custom data doesn’t match the layout, stored file format etc. Take a look at the creation of the FaceLandmarkDataset in the linked tutorial.
The basic idea is to load all paths, store the transformations etc. in the __init__, return the number of samples in the __len__, and load, process, as well as return each sample and target in the __getitem__.
The common approach would be to store the image paths in the __init__ with the corresponding targets and load each sample using the index in the __getitem__.