If you want to preserve the client IP, you can define a virtual server with a type of forwarding IP, with a fastL4 profile. The destination for the virtual server should match what traffic you want to forward through the BIG-IP. To forward traffic destined for any IP address, select network with an IP and netmask of 0.0.0.0/0.0.0.0. If you wanted to only allow traffic to a specific subnet, you could configure that as well (example: 192.168.100.0/255.255.255.0). To ensure the source address is preserved, don't enable SNAT.
If you enable this virtual server on all VLANs, the BIG-IP will forward traffic from/to any host/network. You can restrict which clients can connect to this virtual server by enabling the VIP on specific VLANs. You can also use IP filters or an iRule to add more granular restrictions.
Aaron