Markus,
You will need an initial balance for the location to be active. You could make a small purchase of $9 for 25 voucher credits. Credits are valid for any location, you generate the vouchers by using up these credits, and assign each voucher to the desired location. See: help.hotspotsystem.com/faq/vouchers-faq
Once a voucher has been generated for a specific location it can be used on a different location but the user will need to initially login/activate the voucher from the original location. Existing voucher cannot be modified. See: http://help.hotspotsystem.com/knowledgebase/roaming