Hi @alexander.langenstein!
Take a look at this AU class by Joel O'Connell (page 8 of the handout is about editing xmls): https://www.autodesk.com/autodesk-university/class/Advanced-Customization-Pipe-Networks-AutoCAD-Civi...
Basically, you modified the "AeccPartParamCfg.xml" which declares the variables. Now, you may modify the "AeccSharedPropertyLists.xml", so you can include a list of options that is available for all structures and pipes of the catalog. Or another xml file to include the list to a specific part (make sure to create copies of the original xmls).
The first value of the list is your default. Figure from the handout, as example:

Here, the default (item 0) is nothing, you can type something like "see structure properties". Item 1 is "Reinforced Concrete"... and so on.
Take a look at the handout I mentioned above, valuable information there. Hope it works!!
[PT-BR] Se essa postagem foi útil para você, clique em
Curtir! Se a resposta resolveu o seu problema, clique em
Aceitar Solução. [EN-US] If it was helpful to you, click on
Like! If the answer solved your problem, click on
Accept Solution. 
Matheus Lima Civil Engineer | BIM for Infrastructure Enthusiast | BIM Manager |
Instagram @bim_infra |
LinkedIn