Service Activity with optional resources

Hi all

We need to create a service activity that allows us to book meeting romos. no problema so far. However, we need to offer optional resources, for example a projector, cattering, etc.

Is there a way in Dynamics CRM 2013 to have optional resources? All I can see is to select all or a determinate number, but I do not see an option to create something like "select 0 or more from..." when creating the service.

Any help will be greatly appreciated.

Regards, Ariel