João,
Yes, parametric objects and even entire buildings are possible with GDL, ArchiCad's programming language.
GDL Resources:
Visit this forum's "Libraries / Library Parts / GDL" Topic
Refer to the GDL Reference Guide under the Help menu.
The classic e-book
"GDL Cookbook" by David Nicholson-Cole of the University of Nottingham.
Olivier Dentan's
webpage has some amazing parametric objects.
Visit
this page for examples of parametric buildings using GDL by the Architecture students at UN.
Mark