The SP 26 version of Process Integration Test comes with a great new feature for all those people, who ever regretted that it is not allowed to change an existing message exchange in the test dataset or who experienced the need to upload an own message exchange into PIT.
First of all, in order to clarify it – no, editing of message exchanges in test datasets is still no allowed. But with a new entity
test data draft you can compile you own test datasets with custom message exchanges.
A test data draft contains one or more message exchange drafts. At first, you define the initial version of your message exchange draft creating it from scratch, or you take over an existing message exchange from a test dataset as template and modify it. In the next step, this message exchange is processed by the PI pipeline to get a complete and valid
message exchange with incoming (Before Mapping) and outgoing (After Mapping) version. Once you are satisfied with the results, you can create a test dataset with this data, assign it to a test case and use in your test case run configurations.

New feature in SP 26: Test data draft workbench
If you are curios to learn more about the power of test data draft workbench, checkout the blog post
'Test Data Draft Workbench in Action' and the
documentation in SAP Help Portal.
The blog post
'Towards test-driven development with Process Integration Test' shows, how test data draft workbench can help you during the development phase.
More Information about PIT can be found in
SAP Help Portal and in other blog posts out of the PIT series:
SAP Process Integration Test Tool (PIT) – First Steps
SAP Process Integration Test Tool (PIT) – Create and Execute a Test Case
SAP Process Integration Test Tool (PIT) – What is new with SP 24?
SAP Process Integration Test Tool (PIT) – What is new with SP 25?