\starttext
\setupexternalfigures[location={local,global,default}]
\defineexpandable\ColumnA {30mm}
\defineexpandable\ColumnB {40mm}
\defineexpandable\ColumnABi {\the\dimexpr(\ColumnA + \ColumnB)}
\defineexpandable\ColumnABii {\ctxlua{context([==[\ColumnA + \ColumnB]==])}}
\newdimen\ColumnABiii \ColumnABiii=\dimexpr(\ColumnA + \ColumnB)
A \ColumnA% ok
B \ColumnB% ok
ColumnABi: \ColumnABi
ColumnABii: \ColumnABii
ColumnABiii: \the\ColumnABiii
\externalfigure[cow][width=\ColumnABiii]
~
\page
\stoptext
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