If you need to prepare your file for import, here's a tip on doing so in Google Spreadsheets & Numbers. We haven't tested it in Excel, so let us know if you have success with it.
The Formula used in this example is =A2&"/"&B2&"/"&C2 You will need to use your own Column names, so if you have Month in R, Day in F and Year in C, you would use the formula =R2&"/"&F2&"/"&C2.
The quotes put other information between the data you're merging, so if this was a merge for (A) first name, (B) middle name, (C) last name, you'd put =A2 & " " & B2 & " " & C2 Or if you were merging (A) Height Feet and (B) Height Inches, you could put =A2& "'"& B2& """ Play around with it and you'll be a pro in no time.
If you need more assistance with your spreadsheet program, check their help section.