您是否正在考虑这样的事情?
\documentclass{article}
\newcommand{\plusdots}{%
\mathbin{%
\sbox0{$+$}%
\vcenter{\baselineskip=.3\wd0 \lineskiplimit=0pt
\hbox to \wd0{\hss.\hss}
\hbox to \wd0{.\hss.\hss.}
\hbox to \wd0{\hss.\hss}
}%
}%
}
\begin{document}
\[
a+b\plusdots c
\]
\end{document}