The original first weight tensor is probably of size
[out_channels, in_channels, kernel_size, kernel_size] = [64, 3, 3, 3].
You could initialize your weight matrix using simply the original matrix repeated 2 times :
conv.weight = conv.weight.repeat([1, 2, 1, 1])

The new weight tensor will be of size [64, 6, 3, 3].
The only possible issue is, the original weights are trained with RGB images, whereas you will use it with IR images. But I don’t think it is a problem.