config

first you need an renew command in the config file like that:
\renewcommand{\labelenumii}{\theenumi.\arabic{enumi}.}
\renewcommand{\labelenumii}{\theenumi.\arabic{enumii}.}

after that you can use it in the actual latex files.

to use it in the latex file you have to put this in it:
\begin{enumerate}
\item Test
\item Test
\begin{enumerate}
\item Test
\item Test
\end{enumerate}
\item Test
\end{enumerate}

It will look like that:
1. Test
2. Test
2.1 Test
2.2 Test
4. Test

If you want to have more steps deeper you can add more \begin{enumerate} and \end{enumerate} in the listing.

If you want to have other symbols instead of numbers you have to define it in the config file and then you can use it also in the normal latex files.