When you say "merging a database in to InDesign", are you using the Data Merge feature of InDesign, or are you placing the Excel text into InDesign using either File/Place; or cutting and pasting from Excel to InDesign. If placing/pasting, I would use a GREP search. If using Data Merge, merge to a new document and again use a GREP based solution, unless you are merging directly to a PDF, and in that case I'm not sure how to do this besides manipulating the original data.
You can also split the zipcode in Excel using a string function and make it two fields in ID's merge. But post-merge to a new ID document, you can also use a grep find/change as Colin suggests.