Sorry for the late reply, I think I was on vacation when you posted this. Dictionaries can only manipulate data from one field at a time so they can't read the data from two different fields and combine them together.
If you have access to the IOM API this is a very easy customization to write.
Let's say you want to combine first and last name into one field. In the before or after dictionaries section you could do something like
Import.Fields.GetByExcelName("B").Value = Import.Fields.GetByExcelName("B").Value & " " & Import.Fields.GetByExcelName("C").Value