Have you tried this?
vgg11 = torchvision.models.vgg11()
vgg11.load_state_dict(torch.load('path/to/your/saved/vgg11/weights.pt'))

vgg11_feature_detector = vgg11.features
new_conv_layer = nn.Conv2d(num_in, num_out, kernel, stride)

my_net = nn.Sequential(*[vgg11_feature_detector, new_conv_layer])