Try to pass the tuple in a list as seen here:
dataset = datasets.MNIST(root='data', transform=transforms.ToTensor())
print(len(dataset))
> 60000
sample = dataset[0]

concat_dataset = torch.utils.data.ConcatDataset([dataset, [sample]])
print(len(concat_dataset))
> 60001
print(concat_dataset[60000] == sample)
> True