You can use the Conditional Function IIf and the Text Function, Len, to check for the length of the zip. Then you need Substr to get the substrings from the zip code. The expression may look as follows:
= IIF(Len(Fields.Zip) > 5 ,
Substr(Fields.Zip,0,5) + "-" + Substr(Fields.Zip,5,4),
You can also check the attached report which demonstrates the approach.
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