From early on, GeneXus has provided a generator for iSeries (AS/400). Originally it was Cobol and later RPG was released as well.
The same arguments mentioned for the conversion from GUI to WEB are applied to the "green screen (*)" case. The main difference doesn't lie in the "target" of the conversion, which in both cases will be the Web, but in its "source".
Application development in a centralized environment has some characteristics or features that somehow must be solved in the WEB environment. There are even many variables in this type of conversions, for instance, keeping Db2/UDB on iSeries as the database server makes it possible to maintain part of the solution in a centralized manner or define a more incremental conversion strategy by combining environments.
(*) even though there are also "green screen" solutions for DOS (FoxPro, Clipper based on DBFs) they are not analyzed here, and "green screen" means specifically "solutions centralized on iSeries in RPG or Cobol".
Links of interest
Following you will find some conferences with information that may be useful in this conversion type.