I think you are asking whether or not you can statically map a port from the the dynamically assigned IP Address on the outside interface to a port of an address on the inside network? This can be achieved with the command:
static (inside,outside) tcp interface 80 insideipaddress 80 netmask 255.255.255.255
This would transfer any traffic that hits the interface on port 80 to your webserver on the inside network with address 'insideipaddress'
The issue here is that you need to allow the traffic into your outside interface through the access list, and the last time I checked, you couldn't use a keyword like 'interface' in an access list like you can in the static command, but actually have to specify the ip address of the outside interface. Obviously this is fine if your dynamic IP address tends tends to remain the same over a period of time as some do (for instance some ISPs issue a dynamically assigned fixed IP address), but if not then you would have to change your access-list statement every time your ip address changes, which is a pain.
Also, you should be aware that if you are not a business customer with a fixed IP address, your ISP may have some clause in your contract where you are not allowed to host servers on the service they are supplying - they tend to prefer you to pay the extra money for a static IP address :-/
HTH
Kev
