Use the template feature.

 Use pandoc -Dlatex>template.tex to get template.tex and add \widowpenalty=10000 \clubpenalty=10000 to it before body. To use the template in pandoc. 

pandoc -V documentclass=memoir input.tex --latex-engine=xelatex --template=template.tex -o output.pdf

Hi

How do I prevent widow/orphan lines, tex to pdf conversion. (pandoc -V documentclass=memoir input.tex --latex-engine=xelatex -o output.pdf) 

Thanks,

Magesh