There is no way "Montage to Stack" will work in the ImageJ Macro node, since it requires the input image and the result image to have the same dimensions. The "Montage to Stack" command will however inevitably add an additional dimension (stack) to your image, and in addition will change its x and y range.
You can however achieve what you want with KNIME Image Processing without requiring ImageJ macros. I'll follow up with a small example workflow soon.
Jan