You can create a new VLAN on the interface in question.
Go to System > Network > Interfaces and select Create New
Enter a name like "General Access"
Select Type VLAN
Select the interface which is connected to the switch and enter the VLAN ID (like 10)
Set the Addressing Mode and IP as needed
Activate Ping at least
Now if you go to Policy & Objects > Policy > IPv4 and create a new Policy you can select your VLAN like any other interface.
You might want a policy like
[ul]Incoming Interface: InternalSource Addess: AllOutgoing Interface: "General Access"Destination Address: AllService: AnyEnable NAT as needed[/ul]