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
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.