You might be looking for the option detect-all, see section 5.1 in the documentation for siunitx.
Note that you can always locate the documentation using the command-line/terminal by writing texdoc siunitx, and that works for any package. The documentation is also available at http://ctan.org/pkg/siunitx
Note that you probably want some consistent settings for your typesetting, including numbers. Therefore, assigning your options for siunitx globally would be better than using the settings for only that numbers. Of course, you can use your macro for this, but I really don't see much point in it, as the macro \num{1200} would be plenty.
Therefore, using the command \sisetup{options} is advisable, as both Mico and egreg has pointed out.
Output

Code
\documentclass{article}
\usepackage{siunitx}
\usepackage[default]{lato} %Any other font can be used.
\sisetup{
detect-all,
group-separator={,},
group-minimum-digits=4,
}
\newcommand\mynum[1]{\num{\the\numexpr(#1)\relax}}
\begin{document}
\Huge %Just to get the large sized letters
1,200 %This produces text in lato font
\mynum{1200} %Also produces the text in current font
\num{1200}% The same as above, but now extra macro-definition is needed.
\end{document}
\sisetup{detect-all, group-separator = {,}, group-minimum-digits = 4}and then use\num{1200}? – Manuel Sep 17 '16 at 12:21$\mathsf{1{,}200}$in the same document (which is whatsiunitxis doing, in the end). – Joseph Wright Sep 17 '16 at 12:53\mathsf– egreg Sep 17 '16 at 13:03