\documentclass{article}

\usepackage{xfp,etoolbox}

\newcommand{\newreg}[1]{\csdef{reg-#1}{}}
\newcommand{\setreg}[2]{\csedef{reg-#1}{\fpeval{#2}}}
\newcommand{\addreg}[2]{\csedef{reg-#1}{\fpeval{\csuse{reg-#1} + #2}}}
\newcommand{\valuereg}[1]{\csuse{reg-#1}}

\begin{document}

\newreg{myx}
\setreg{myx}{0.0}
\addreg{myx}{0.5}
\addreg{myx}{-0.25}
\valuereg{myx} % this expands to 0.25 (0 + 0.5 - 0.25)

\newreg{abc}
\setreg{abc}{-2 + 3.1}
\addreg{abc}{5.1 - 1}
\valuereg{abc} % this expands to 5.2 (-2 + 3.1 + 5.1 - 1)

\setreg{def}{1 + 2 + 3 + 4} % 10
\addreg{def}{\valuereg{def} + 10} % Add 20 (10+10) = 30
\valuereg{def} % 30

\end{document}