Wireless Access Point – Convert Wireless ADSL Modem Router into a Wireless Access Point

If your ISP can send multiple IPs through the circuit and you get a cheap switch, then you'd pull IPs in parallel by static assignment, keep in mind there is no firewall and your IPs are 100% public with this option. If you turn off DHCP and NAT, then you should be able to use that ADSL modem/router as the switch for this scenario.

If your ISP is only sending you one IP and you can't get more than you must use DHCP/NAT, hence you must use a router.

If the router has both WAN and LAN ethernet ports, you can usually disable DHCP in the router and change the configuration to move the wireless router's network configuration to some unused IP range, then plug your network into the LAN side of the router and nothing into the WAN side to make a poor man's access point.

Wireless clients will connect to the wireless router and receive an address from your main DHCP server (along with the main gateway, dns servers, etc) then communicate with the rest of the network without using the "router" part of the wireless router, as if they had been plugged in to a switch or hub on the network (which is what the LAN ethernet ports are).