\documentclass{article}
\ExplSyntaxOn
\tl_new:N \l_alan_cols_tl
\newlength{\mylen}
\NewDocumentCommand {\makecol}{m} {
\tl_set:Nn \l_tmpa_tl {\the #1}
\tl_set:Ne \l_tmpa_tl { { \exp_not:V \l_tmpa_tl } }
\tl_put_left:Nn \l_tmpa_tl {p}
\tl_put_right:Nn \l_alan_cols_tl {\l_tmpa_tl}
\l_alan_cols_tl
}
\begin{document}
\setlength{\mylen}{30pt}
\makecol{\mylen}
\end{document}