Advanced Roadmaps - Issues from different projects

Hello,

I have a Jira project called "PBI". This project contains epics which must go through a workflow designed for this project (related to approving these epics).

These epics have child issues (stories, tasks) in different projects (DENIK, CTP etc.). When I create a plan I would like to see list of these PBI epics with their children. 

I have a filter for this plan which says 'project = "PBI" AND issueType = Epic'. I know that to see the child issues related to these epics, I also have to include them in the query. But this seems to be impossible because what would need a subquery which would say "select also tasks or stories which have a parent in project PBI".

I know there are some extensions that would help me to create such a subquery, but that is an extra cost to Jira Premium and even those plugins are not real-time (meaning that when you update some issue, it takes some time to reindex the filter).

Is there another way how to achieve that just using Jira Premium? It should result in this (displaying epic with children from different projects, not only epics), but without external plugins.

Thank, Tomáš