How can I combine two source fields to generate one destination, such as UD2?

this should look much like the golfstream debits and credits parser rule. Essentially, you'll want to test if the supplier number field is empty, if it is, use the supplier name. I am attaching code below that is untested but should hopefully point you in the right direction. This logic would be called on the Ud2 data source dimension in either a complex expression or parser business rule
'if the supplier number is empty, return the supplier name
If String.IsNullOrEmpty(args.Value) Then
'zero based index - get the supplier name column and column value
Dim supplierNameColPosition As Integer = 3
Dim supplierName as string = api.Parser.DelimitedParsedValues(supplierNameColPosition)
Return supplierName
Else
Return args.Value
End If