How can i insert an Intelligent Mail Barcode into Pages 09 document
One way you could achieve this is a multi-part batch process, it very much depends how you need to generate your barcode though.

Do you need to from Address to barcode?
Are the addresses clean and normalised?
Do you already have, or can generate the barcode identifier?
The better your data is the easier it'll be to put this together.

If you don't have the barcode identifiers already is it possible to get a third party in your country that does address validation and partners with the postal services data to clean and add these so you've got valid addresses + barcode identifiers returned.

Assuming you can do this process, either via a third party or software, to get 
barcode identifiers into your source spreadsheet.

I'd do a multi-part mail merge, i.e:


Run the spreadsheet against a compliant barcode generation tool e.g. ID Automation's Image Generator output your barcodes in batch, with a unique name to a named folder.
In your Pages mail merge include the relevant barcode image. 


Other options are using a barcode font, you've got to be careful with the source data you pass in to make sure it's a real valid barcode e.g. IDAutomation's font

I've only recommended paid software, likely you can do the generation cheap-free in the open source world but make sure you get your barcode checked for compliance before committing to the full print run!