How to concatenate two files?

I’m running remarkjs from within hugo. The markdown file is processed through the sourceUrl parameter as follows:

 var slideshow = remark.create({
          highlightStyle: 'monokai',
          highlightLanguage: 'remark',
          highlightLines: true,
          sourceUrl: "/slides/{{ .Params.slidesfile }}"
        }) ;

where .Params.slidefile is the name of the file. What I need to do is concatenate content to the file prior to processing, something, say like Is there a way?