I have a n-based router from Linksys with DD-WRT installed. I would like to turn this in to an access point because I already have a router/firewall installed on my network.
You have a few options:
1) two networks, wired and wireless, routing between them:
You need to:
and then you're mostly done. Note that while step 2 is fairly straightforward using dnsmasq, the difficult of step 1 can vary from 'trivial' to 'you need to reverse engineer the wireless drivers'. Giving you more instructions at the moment requires more info (model number & rev) of the Linksys router in question. There's a DD-WRT table of supported hardware that can probably help you.
Oh, and all this is presuming that your definition of 'access point' is something like 'a way to connect a machine with a wireless ethernet card to the network' as opposed to some kind of walled garden setup.
2) one network, using a LAN port to bridge from wired to wireless:
3) one network, using software to bridge from wired to wireless
The trick is to make sure DHCP is turned OFF in the router/access point so that your wireless clients get IPs from your existing router/firewall.
There are more details on the DD-WRT wiki but the main idea is that you set the WAN to "Disabled", Disable DHCP, and plug the AP into your network using one of the LAN ports instead of the WAN port (or use the "Assign WAN Port to Switch" feature).
You can actually do this with basically any Wireless router without DD-WRT using the same basic steps (disable DHCP, plug into LAN instead of WAN).
You can still serve DHCP on the wireless access point as long as you set the DNS & default gateway addresses to your router's IP address and you don't overlap the DHCP IP addresses you are serving on the router & access point(s). This will allow for quicker IP address assignment when a network client is connecting to the access point.