I found the solution. It's actually quite simple. Because it's data blending on DATE, I can create a calculated field from Experience and tie that to the new metric as follows. Data Blending allows below flexibility even if you do not have an attribute in a given data set.
IF ATTR([CustomSQLQuery (DBMortgage)_1].[Experience])='Web' Then Sum([UUs OLD])
ELSEIF ATTR([CustomSQLQuery (DBMortgage)_1].[Experience])='Mobile' Then NULL