Help with DHCP on VLAN Interfaces

Hi all. I am relatively new to fortinet and would appreciate any assistance on the following matter.

Ive created an SSID "RadiusWifi"  which authenticates users via a RADIUS server. This SSID is also set up to dynamically assign the connected user to their designated VLAN as configured on the RADIUS Server using WPA2 Enterprise. I acheived this by creating all of the VLANS with 'Manual' IP addressing and configured a DHCP Server on each . I  assigned each of the created VLANS to the "RadiusWifi" interface. Users are currently able to connect and be correctly assigned to their designated VLANs. 

My problem is... I would like it to be set up so that each of the authenticated users  get IPs from an external DHCP server when they connect rather than using the ips provided by the fortigate VLAN interface's DHCP.

The users typically connect to the network via a wired connection which recieves ips from the external DHCP . My intention is for the "RadiusWifi" SSID to be an extension of the users wired connection. As a result, they would be assigned ips from the same subnet they typically connect to on a wired connection.

Can this be done?

Am i approaching the issue in the wrong way? are there alternative methods for dynamically  assigning these users their designated subnet? (example: A possible RADIUS Server Configuration)

Any assitance will be greatly appreciated.

Cheers.