Assuming you're using a label merge and your data have already had whatever pre-processing might be required to get it into the barcode's syntax, it's as simple as applying that font to the mergefield. To do that:
• select the mergefield
• press shift-F9 to expose the field code, which should look like: {MEREGFIELD Barcode}
• add the CHARFORMAT switch to the field code, thus: {MEREGFIELD Barcode \* CHARFORMAT}
• Format the M in 'MEREGFIELD' with your barcode and whatever point size you want it to be
• Use the 'update labels' button to propagate the change to all labels
• run your mailmerge.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]