It shouldn't. You're simply adding a tag, on top of the other normal traffic, on the access point port or switch port.
Here's mine, VLAN 8 on a 24 port switch, to connect access points back to pfsense. I'm using VLAN 8 for a guest network, and the access points support VLANs and multiple SSIDs. The guest network is running on top of the LAN network in pfsense, and the guest network is setup with its own subnet. Everything works perfectly. In my picture, port GE27 (back to pfsense) would simulate your port 1 on the Netgear.
screenshot765998.png
I'm assuming the DD-WRT box you're got will behave the same way.
Sorry, I forgot, your port 8 on the Zyxel also has to be tagged with your new VLAN number.
So, quick summary - add a new VLAN to pfsense, parent interface is LAN, tag port 8 and 2 on Zyxel with your new VLAN number. Then finally, tag port 1 on Netgear with the same number. Tagged and untagged ports on networking gear can exist at the same time, if the gear is any good.
Jeff