
\documentclass{article}
\usepackage{fontspec}
\setmainfont{Libertinus Serif}
\usepackage{stackengine}
\pagestyle{empty}
\begin{document}

\Huge η̣ ḍ

\newcommand*{\doteta}{\stackinset{c}{0.15\fontcharwd\font`η}{b}{}{ \symbol{"0323}}{η}}

\newcommand*{\dotd}{\stackinset{c}{0.2\fontcharwd\font`d}{b}{-0.32\fontcharht\font`d}{ \symbol{"0323}}{d}}

\Huge \doteta{} \dotd

\end{document}
