One of my clienst has a german quite far designed joomla/k2 site.
I ould like to use takai or other templates to certain parts of that site and want to install the demo content.

I am quite new woth your templates and learned succesfully complete new install of demosites as well as the basic raw template

But in order to save time and configure the template same as demo, I would prefer to install it over or in an existing installation without overwrite existing dabase.

Any hints for me?

That is not possible I am afraid.
There will be a conflict in your DB if you try to merge the tables.