No. Not possible. Once you bundled multiple ports into a hard-switch, your are no longer be able to refer to individual physical port to create a VLAN. A VLAN can be bound to the hard-switch. In your case, I would leave those 5 ports separated, then create VLANs on the port you need to have them.
I guess now I see what you meant. With FGTs, you can't share a non-tagged interface with multiple ports without a hard- or soft-switch. So the 4 ports other than the up/downlink port need to be in a hard-swich to use as one interface.
Basically FGT doesn't have a concept of native vlan although you might see the word in some document outside of the handbook/admin guide. The untagged interface is never VLAN interface, and is always the parent interface/port where you might attach multiple VLANs on. In other words, there is no "access ports".
So to change "native vlan", you need to let the switch do that prt, either the core switch or another VLAN capable switch. If the former you choose, it physically looks like a firewall on a stick. Only the uplink port(ports if you do LAG with the switch) is connected to the core switch.