Custom dial plan is done with a “Custom Destination”. For your example above, you would create a custom destination with a Goto string of:
door-intercom,900,1
and then use this destination in the GUI to direct the call. If you need a dialable feature code, use a Misc Application.
However, your dial string is not going to work as it’s mis-formatted. You also need to look at the help for the Dial application, and the option to pass DTMF post answer is with the D option, so it will look something like:
[door-intercom]
exten => 900,1,Dial(Local/220@from-internal,30,D(www1212))
exten => 900,1,Hangup