Question

kan anyone tell me if/how its possible to predefine some default values when creating a product.

Like: default taxclass, product is “visible” and “in stock”.

I hope someone can help me.

Ik have magento 1.6.2 CE

Answer 1

Probably via config files.

Try, in app/etc/local.xml (or in any config.xml in local/your_package/one_of_your_module/etc/config.xml:

<default>
    <cataloginventory>
        <item_options>
            <is_in_stock>1</is_in_stock>
        </item_options>
    </cataloginventory>
</default>

Not sure about tax_class_id and visibility, but you may try & go with config.xml as well in order to set or change default values.