Ah sorry, i might have read your question wrong.
The thing i can think of is dirty but effective.
You can run a query like this:
$writeConnection = $resource->getConnection('core_write');
$writeConnection->query("UPDATE `catalog_category_product` SET `position`= 10000 WHERE `category_id` = 23");
I have tested the query on my own magento database and it works like a charm.
do note: 23 in this case was the id of a specific category so change that to the category you would like to update with the new data.
After you have run the query run a index of
product flat data else the new data won't show up.
As I said it is dirty doing it this way.
In any other case you could try and write your own module handeling all this for you by simply adding a category id and to what number the position of the products should change to.