Backfill Rollup indices from external Sources

hi there,
we have a legacy selwritten data retention solution which i want to migrate into pure elastic.
it contains daily averages(weighted) af some metrics over the last 5 years.
The Rollup off the latest metric already in Elastic are correct and usable, but however i try, i cant get the legacy data into my rollups.
For testing purposes i fetched a dataset for one day directly from the rollup-index with _search, removed the _id field and set the timestamp.value field back to an older date that has no rollupdata so far and wrote it via bulk write directly into the rollback index.
i can query it via normal _search queries in the rollup index, but the data does not show up via _rollup_search.
did i miss any conditions or prerequisites to "manually" create rollback data(maybe some metadata regarding rollupjobs)? or is there any docu or examples regarding this topic that i missed?
i only found a report of the guardian describing that they did exactly what i'm trying, sadly without technical details