"the switch's wan port" ...
switch's dont have wan ports. Routers have wan ports.
I'm guessing what you want to do is something like this:
int fa0/0
ip address dhcp
no shutdown
ip nat outside
exit
int fa0/1
ip address 192.168.0.1
no shutdown
ip nat inside
exit
ip nat bla bla bla (configure a dhcp pool, assign it to your fa0/1).
Although, there's a chance it wont work. For example, the 850 series routers, have one with ethernet WAN, one with DSL wan, one with 3G wan... etc...
I'm not sure off the top of my head that the 2620 supports a DSL wan.