Missing layers when converting from .caffemodel to .onnx

[infinityp913]

Input: .prototxt file, .caffemodel file

Output: .onnx file

Problem

The caffemodel file has certain layers like, "Silence", "Slice" , "Split" layers (that are defined in caffe.proto so not custom layers) which don't show up in the converted onnx file.

Is it because Silence, Slice and Split aren't among the supported operators? But, Eltwise layers are also missing even though it's listed as a supported operator.

If I wanted to add support for those layers, could someone brief me how I should do that?