Hi Jeremy.
As long as you are using the full area code and not using local dialing for the UK it is easy.
American numbers are 10 digits in the format of a 3 digit area code followed by 7 digits beginning with 2, 3, 4, 5, 6, 7 & 8 so don't clash with national UK numbers.
Set up multiple dialing rules 92, 93, 94, 95, 96, 97, 98. They should all expect 9 more digits (this cover all American number prefixes.)
Set this to go out over a new route/route list using your trunks with a new digit modification.
Set the digit modification to absorb the 9 and add in 001.
The actual outbound number sent will be sent in the International format +1-XXX-XXX-XXXX.
You can always then use COR to restrict this to only the USA extensions.