Hi,
You can indeed restrict "Purchase order" method to a specific user group.
But to set a user group if the customer pay with a specific payment method, there is no options for that by default in HikaShop.
If you have PHP knowledges, everything can be possible. The creation of a plugin based on the function "onBeforeOrderUpdate(&$order,&$do)" could do the job, if you check the payment method and if the order is confirmed, so change the user group.
Here is the documentation related on the plugins:
www.hikashop.com/support/support/documen...mentation.html#order
ps: if you are not a developer, you can still post a request in the commercial jobs section of this forum.