For Oracle you can use the Oracle Data Integrator to import HSQL databases (the format of the Mendix Runtime built-in database). See this full reference (the chapter on Hypersonic SQL).
An easier option for non-Oracle experts is the HSQL Transfer tool. This tool can also be used to convert HSQL data to PostgreSQL. See the documentation of this tool for more information. I have no experience with using it, so more info is welcome!
And the other way around? Is it possible to convert a Postgres or Oracle database into a HSQL database?
It would be really nice to have some sort of export/import function for data which does not rely on a specific dbms implementation or it's custom import/export format which cannot be reused for other rdbms systems! A complete description of the database structure is available in the metamodel, which is used by the Mendix server already to manage the database structure. So a dump only needs to contain the data objects and associations between objects.
This way you won't need a full mesh of options to convert between all possible db-systems.
@Jornt I think you can also use the HSQL Transfer tool for that. See the documentation of this tool for more information. I have no experience with using it, so more info is welcome!
I am trying to do this exact thing now but I have found that the link for the HSQL Transfer tool provided in this thread is broken... Anyone have any ideas where I can find it now?
I came across this post trying to achieve this myself. In the current versions this is possible by following these instructions: https://docs.mendix.com/howtogeneral/tips/migrating-your-mendix-database
I was able to convert my built-in database using the following settings under the custom tab:
Use the database tab to configure your target database.