I have copied and modified the single.php file as decribed below:
http://codex.wordpress.org/Post_Types
in order to add a custom html file to it.
is it possible to create a post with the normal procedure on the dashboard but that would call that new single-{posttype}.php instead of the single.php ?
meaning being able to choose with template to use from each time .
Well, there is a plugin for that:
Otherwise, you may want to check the filter hook
template_include.