Adding a leg
 

Ive got DUB-EWR-MCO booked on 7/1/12. I wanted to add on MCO-LAX on the 11/1/12. Is this possible after I made the initial booking and am I just paying the taxes? Could someone calculate the taxes for the MCO-IAH-LAX with CO for me please?

Also, Is DC open on Christmas Eve?

you may have problems getting them to accept the routing as a single trip as a stopover is normally a stop as part of your route

you currently have DUB-MCO and DUB-EWR-MCO is a valid routing
you want to change to DUB-LAX and DUB-EWR-MCO-LAX is not showing as a valid routing, there are a number of DUB-LAX options but none show as via MCO

In theory if they would accept the routing it would be taxes plus change fee, you can buy a revenue ticket for your date on United for £103 (direct, no need to go via IAH)