I don't really understand how you meant that.
Let me give explain in more detail:
You will have the default vmbr0 with probably the first interface as bridge port.
You can then add another vmbr (vmbr1) using the second interface as bridge port. This will be the WAN port connected to your modem.
vmbr1 does not need to have any IP configured on the bridge itself.
The simplest solution then is to add two NICs to your virtualized pfSense. The first with vmbr1 as bridge and the second with vmbr0 as bridge which is connected to the switch on the internal network.
If you want to split up your traffic more you can add another bridge with one of the unused interfaces and use that for the internal interface of your pfSense.
Code:
vmbr0 ---- NIC1 ---- switch
Modem ---- NIC2 ---- vmbr1 ---- pfSense WAN
pfSense LAN ---- vmbr2 ---- NIC3 ---- switch
I hope this is understandable.