The original post seems to want to have a Pega app make updates to the IMS db, not use it as the back end for Pega. The support guide is referring to Databases Pega supports as the back end for the Pega server, as far as I know that does not mean Pega can not use a Database rule to connect to an unsupported DB in an activity for instance. I haven't tested this so please let me know if you know otherwise.
Sorry for the confusion. For the IMS database, I'm actually looking to read particular information from it, not necessarily make edits or add to it. Technically I am using it as a backend for information. Does what you said still apply?
Yes what I said does still apply. When I referred to the "Pega back end" I was referring the database that Pega is configured to use to store rules and case data. Your use case is more what I would call reference data or an integration point, though I can see the confusion as "back end" can be used in both contexts. I would note that what Paul said about this being unsupported, it may work but it won't be supported and could break with an upgrade as it isn't supported.
Pega supports connecting to external databases via a JDBC connection. Since IMS has a JDBC driver I would assume Pega can connect to it. You will need to create a Database rule with the connection information and the use the "External Database Mapping Wizard" to create a mapped data class. If you have a mapped class you can use reports and lookups (via Datapages) to get data and Obj methods in activities or Connect-SQL rules to read and write to the tables.
That is correct. But, Pega still adds no support for accessing IMS.
You can take Sybase's driver and place it in Tomcat or WebSphere, use a JNDI reference and get connected. However, should you encounter issues, Pega does not support it. For example, PEGA converts schema names to uppercase and Sybase has case sensitive naming. If you run into this, you can't expect Pega to provide a hotfix to address it.