Hi Annika
You can't add the wbse in cost element master data as far as I know.
Option 1
When you maintain the settlement receiver as GL acct, double click on it and you can maintain the additional details in settlement rule like cost objects. Here, wbse should be available.
Also note that you need to implement the corrections in sao note 448340 in order to pick cost object from settlement rule
Option 2
Else, you can also write a substitution in okc9.
I prefer option 1
Br. Ajay M