Hi, we have a working installation of Phabricator and we want to create a new repository via API. Is there a way to tell the API that we want a HOSTED and not an IMPORTING repo?

There's no such parameter and according to the API answer it's automatically an IMPORTING ONE:

[isActive] => 1
[isHosted] =>
[isImporting] => 1
[encoding] =>
[staging] => Array
    (
        [supported] => 1
        [prefix] => phabricator
        [uri] =>
    )