Buy EGM (Enterprise Gateway Manager):
With this product you can virtualize a DB2 database as an Informix
database on a virtual informix server. Then joining is easy.
Note: The query is running on IDS
Configure the DB2 database as a federated server and create a nickname
If you first install Informix CSDK on the DB2 machine and then install
Informix, then the DB2 wrapper is installed automatically.
After that just specify the database server as a FEDERATED instance
and you are able to create servers, user mappings, nicknames etc.
Note: The query is running on DB2
The last solution is free of charge I believe due to the fact that Informix
now belongs to the DB2 family. This is also possible with other databases (like Oracle) but then you need the Websphere Information Integrator product.