Hi, in my opinion when accessing to mainframes there are three approaches:
1. The mainframe has some kind of external access configured (JDBC, Web Services, etc) In this scenario Denodo can access the data in the mainframe by using its out-of-the-box connectors (for example, is it in a DB2 database?)
2. A third-party software that acts as a bridge to external applications is installed in the mainframe (usually these connectors provide interfaces for external apps such as JDBC, ODBC, Web Services...)
3. The data is extracted from the mainframe and loaded somewhere else. I'm not happy with this approach because it is not an architectural best practice.
In any case, if it is not possible to use one of the out-of-the-box connectors, you will have to create a custom connector (custom wrapper) to get the data (read the "DEVELOPING CUSTOM WRAPPERS" section of the Virtual DataPort Developer Guide for more information about custom connectors)
Hope this helps!