Hi Kahn,  I guess you are looking to use the pre-trained weights within the ResNet architecture.   The simplest thing would be to expand your tensor with the input data by replicating the one channel:
# make up some input tensor
input_tensor = torch.randn(batch_size, 1, img_size, img_size)
input_tensor = input_tensor.expand(batch_size, 3, img_size, img_size)
# go on to use in resnet ...