Hi Henning, 

The way that OTRS creates a workflow is with the status. You can create ACLs (in perl) that help you to achieve that. Inside you /opt/otrs/Kernel/Config/files create a new file "MyACL.pm", add these 3 lines:

package Kernel::Config::ACLs;

use strict;
use warnings;

and you can begin to create your workflows with ACLs. In the OTRS documentation there's a little explanation and examples about it. Also in the FAQ from OTRS ( http://faq.otrs.org/otrs/public.pl?Action=PublicFAQZoom;ItemID=68 ) you can see what kind of ACLs you can write.

I think it's the best solution to achieve what you want, I did it also like that, but maybe anyone else knows a better way.

In the OTRS forum  - http://forums.otterhub.org/ - there are also lots of questions and threads about that kind of issues. 

Good luck, and don't hesitate to ask for help!

-- 
Rafa Quiñonero Figuerola

Tlf: 01577-6805709
rafakorp@gmail.com