<p>I'm customizing a SugarCRM 5, and in my <strong>SugarCRM database</strong> I have all invoices which were imported from our ERP. Now, I would like to know if it is possible to create a new sub-panel in the Accounts Panel <strong>without editing the original SugarCRM files</strong>, so that my client invoices index are visible in that interface.</p>

Last time I checked, you could use the
module builder
to extend the interface. From 5.0 (or maybe 4.x) on, Sugar added all those APIs, which should enable you to extend SugarCRM without hacking it in and losing it with the next upgrade.
Hope that helps!