Is there a LaTeX package for units not included in siunits ?

With SIunits:

\documentclass{article}
\usepackage{SIunits}
\addunit\decibelm{dBm}
\begin{document}

\decibelm

\end{document}

I would (not surprisingly) use siunitx. This looks like a separate unit for which I might use

\documentclass{article}
\usepackage{siunitx}
\DeclareSIUnit{\belmilliwatt}{Bm}
\DeclareSIUnit{\dBm}{\deci\belmilliwatt}
\begin{document}

\si{\deci\belmilliwatt}
\si{\dBm}

\end{document}

On the other hand, if it's a qualifier then

\documentclass{article}
\usepackage{siunitx}
\sisetup{qualifier-mode = text}
\begin{document}

\si{\deci\bel\of{m}}

\end{document}