You can't change the jetway type, because they are not generated on the fly, but pre-compiled by us and included in the package as about 900 .BGL files for all airports in the world that have jetways, and the choice between glass/steel is based on some rules based on location, which result not having them in the US, when they are almost never used.
About the add-on you mentioned, real life KMCI has solid jetways, so it doesn't seem very accurate and, in any case, you are not supposed to use GSX replacement jetway on 3rd party airports, they are only meant to improve default airports.
