Writing this function did the trick:
add_markdown_snippets <- function(my_file = '~/.R/snippets/markdown.snippets', append = TRUE){
my_text <- "
snippet sql
{sql, connection = conn, output.var = '${1:df}'} ${2}
"
cat(my_text, file = my_file, ...)
}
Now a user can execute this function after loading the package to update their snippets.