Of course it is possible. But to tell you how reguires a lot more information. Otherwise we have to make presumptions.
A – Exactly what are you wanting from the AS/400? Data from DB2, or event logs, or transactions of some kind?
B – Where and on what is the MySQL database running? If it is running on a ‘Linux’ instantiation on the as/400, then you use inter process communication.
C – If separate platforms, how far apart? Same room? Same building? Same neighborhood? Same city? Same World? The further apart the systems, the more the amount and validity of the data matter.

Blind shot would be to get the AS/400 data into XML pages. Then write a script (PHP) to read the pages into MySQL.