Using the Intersystem Communications Function (ICF) support of the AS/400 operating system you can have an application program (RPG, COBOL, etc) on the AS/400 appear as a terminal to IMS. This minimizes the impact to IMS while making the AS/400 application potentially more complex (as it has to handle errors much like an operator might). A better solution, though potentially requiring a bit of work on the IMS side depending on what is currently in use there, is an application to application interface based on either APPC or TCP/IP. The APPC support is available as part of the ICF support, TCP/IP socket interfaces are also part of the AS/400 operating system and based on standard TCP/IP programming interfaces. These solutions provide real time access to the IMS database and are not difficult to implement.
For batch interfaces you can always fall back on formatting the data on the z side and then using RJE, FTP, etc.
I hope this gives you a general feel,