You can use the toolset campaign plug in to change the number from 4 to 6. However, there are reports that use of the toolset to alter campaign settings will cause subsequent loads of saved games to fail. The workaround for that is to do new game and then cancel at the character selection screen and then load your saved game. Inconvenient but it seems to work.

Is there a possibilty to expand the Number of own created Companions from 4 to 6?

After subscribing, navigate to folder steam\steamapps\workshop\content\2738630\3526867889\override and unzip and follow provided instructions to install.