Yes. Use a condition like

count(xdXDocument:GetDOM("MySecDataSource")/dfs:myFields/dfs:dataFields/d:Group/@firstfield) = 0

You will have to type the expression in yourself by first selecting a field
from your secondary data source when you are in the "Conditional Format"
dialog box, and then selecting "The expression" from the first drop-down list
box, and then typing in the count(...) = 0 expression around the selected
secondary data source field in the text box behind the drop-down list box.
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S.Y.M. Wong-A-Ton