Hi @Yoga,
I’m afraid that this currently isn’t feasible. Exporting content is mostly the only option: Migrate multiple journals into one - #2 by marc
But exporting the content comes with the caveat that it does not contain any of the associated workflow data, unfortunately. As noted in the post linked to above, it is on roadmap for future development, but it will likely be some time before it could be developed.
It was feasible, apparently, to merge two OJS 2 instances, using this third-party tool: Merging a few OJS 2 installations into a single OJS 3? - #2 by ajnyga
I don’t know the status of this tool, however, as it might not be kept up-to-date - and its not available for OJS 3.
Thanks for clarifying your situation. That’s not uncommon for organizations to host multiple instances of OJS, but merging them into one is not something that is easily done (or really feasible, so far as I know).
-Roger
PKP Team