It is not possible to use Report Builder with Custom Data Sources (in SQL Server 2008 R2 or 2012) as per this knowledge base article.
This document describes a workaround that can be used when using "IFS Applications" data source.
Note: Make sure to use schema names
If you have used an embedded Oracle data source instead of a shared data source,
still it is possible to change the data source to IFS Application.
Note: The connections depicted in above screenshots has been created using an IFS Applications instance with Oracle Authentication (DB)
After designing the report and just before saving the report to SSRS, open
the data set properties and change the schema of the query as follows.
The extended server will substitute APPOWNER
for &AO, when the report is
executed.
You can ignore the below error. This error will pop out because, according
to oracle syntax, &AO is not allowed.
Now change the Data source to IFS Applications from SSRS as described
above.