dms.tex

\documentclass{article}

\usepackage{lipsum}

\usepackage{xparse}
\usepackage{pgfmath}

% defaults:
\newcommand*\skillingfactor{16}
\newcommand*\markfactor{6}

\newcommand\moneyadd[2]{%
	\toskilling{#1}%
	\edef\tmp{\skilling}%
	\toskilling{#2}%
	\pgfmathparse{\tmp + \skilling}%
	\todms{\pgfmathresult}%
}

\newcommand\todms[1]{%
	\pgfmathsetmacro\daler{div(#1, \skillingfactor * \markfactor)}\daler%
	.%
	\pgfmathsetmacro\mark{div(#1 - \skillingfactor * \markfactor * \daler, \skillingfactor)}\mark%
	.%
	\pgfmathsetmacro\skilling{int(#1 - \skillingfactor * \markfactor * \daler - \skillingfactor * \mark)}\skilling%
}

\makeatletter
\newcommand\dkm@toskilling[3]{%
	\pgfmathsetmacro\skilling{int(#1 * \markfactor * \skillingfactor + #2 * \skillingfactor + #3)}%
}
\NewDocumentCommand\toskilling{ > {\SplitArgument { 2 } { . } } m }{%
	\dkm@toskilling#1%
}
\makeatother

\begin{document}

With default markfactor 6:

\begingroup
	\toskilling{1.2.3} \skilling{} = 1*6*16 + 2*16 + 3 = 96 + 32 + 3 = 131

	\toskilling{3.2.1} \skilling{} = 3*6*16 + 2*16 + 1 = 288 + 32 + 1 = 321

	\todms{400} = 4*6*16 + 1*16 + 0 = 384 + 16 = 4.1.0

	\todms{452} = \moneyadd{1.2.3}{3.2.1} = 452 = 384 + 64 + 2 = 4*6*16 + 4*16 + 2 =  4.4.4
\endgroup

Resetting markfactor to 4:

\begingroup
	\renewcommand*\markfactor{4}
	\toskilling{2.0.0} \skilling{} = 2*4*16 + 0*16 + 0 = 128

	\toskilling{0.1.2} \skilling{} = 0*4*16 + 1*16 + 2 = 18

	\todms{241} = 3*4*16 + 3*16 + 1 = 192 + 48 + 1 = 241

	\todms{146} = \moneyadd{2.0.0}{0.1.2} = 146 = 128 + 16 + 2 = 2*4*16 + 1*16 + 2 =  2.1.2
\endgroup

\end{document}

This turned into a full package; see https://www.ctan.org/pkg/non-decimal-units