I found a TechNote on SupportNet: How Do I Create Slots in Metal Using Calibre DRC
It's not a detailed step by step method, in fact it discusses some reasons why you may prefer to do the slotting in the routing tool rather than the verification tool.
Hope it helps,
Thank you very much for your hint. I will check it out, once i could login (i registered yesterday, but still haven't received confirmation from SupportNet).
The thing is, i know that it would be better to do in Virtuoso, for example, but this is an older design, and we were thinking of manipulating the gds2 directly in Calibre as more efficient way. Seems to be tricky, though..