I got external references to work with memoir and hyperref here: hyperref and xr-hyper break memoir's \titleref, but references to external documents don't work with memoir \NumToName: Chapter~\NumToName{\ref{chap:example}} errors out with Missing number.
Here are the two files that show this behavior:
\documentclass[12pt]{memoir}
\usepackage{xr-hyper}
\usepackage{hyperref}
\begin{document}
\chapter{Example}\label{chap:example}
Examples
\end{document}
Call that one example.tex, run pdflatex on it, and then the second file references it:
\documentclass[12pt]{memoir}
\usepackage{xr-hyper}
\usepackage{hyperref}
\externaldocument{example}
\begin{document}
See Chapter~\NumToName{\ref{chap:example}}.
\end{document}
Compile the second file and the issue will appear.
Here's the error message I get:
! Missing number, treated as zero.
<to be read again>
\protect
l.7 See Chapter~\NumToName{\ref{chap:example}}
How to I get a NumToName reference to an external document?

\refgenerally isn't (not on theTeXlevel), see Karls solution. – daleif Mar 07 '14 at 13:15