Edit the form and add a sub-report. You'll need a second sub-report for it to work for printing from history too (different tables).
Normally I'd suggest a UDF to the work table, but that won't work for tracking numbers, because there can be more than one (and I doubt the business objects are there for such a UDF). Don't link the tables in the report because that is always complicated with there possibly being zero or multiple matches in the secondary table.

The tracking numbers are in SO_InvoiceTracking. I generally find it easier to create a subreport in the Crystal Invoice form.

Actually the tracking numbers, or at least the first 5 I think it is, are part of the Invoice works table. It is a simple matter of just adding the field to the form.