Support a safer workflow to continue/import historical data for an existing Matomo property on UI

Users migrating from GA4 to Matomo often need historical imported periods and live Matomo-tracked periods to be consecutive for the same business property. Today, GA import creates a separate site, and if an import is cancelled or restarted, users can end up with multiple sites or a workflow that does not match how they need to present continuous reporting.

Current limitation
Existing support guidance indicates GA-imported data is currently imported into a new site and cannot be merged later into an existing Matomo website. This appears to be due to GA import relying on aggregated Google report data rather than raw event-level data.

Requested improvement
Please evaluate a supported product workflow for customers who need historical GA-imported data and ongoing Matomo-tracked data to appear as one continuous property.

This could be implemented in one of several ways:
On the UI:

allow selecting an existing destination site during GA import where technically safe
allow append-only import into an existing site before native Matomo tracking begins
provide a guided “historical data continuation” workflow instead of a true merge
provide a supported migration/linking mechanism that presents imported historical data and ongoing tracked data as one logical property in reporting

Important note
This request is not necessarily “just add a dropdown.” If true merging is not technically possible, a supported alternative UX/workflow would still solve the customer problem.

Customer use case
A customer is gradually enabling Matomo tracking on more parts of a website/application and wants to re-import selected GA periods so that historical imported data and newly collected Matomo data are consecutive for the same site, without having to create or maintain a separate site for the historical portion.

Expected outcome
Customers have a supported way to build one continuous reporting experience during GA-to-Matomo transition on the UI, or a clearly guided alternative that avoids accidental creation of unusable duplicate sites.

The issue names no files, tests, or entry points; begin by tracing the existing GA import UI and how it creates destination sites. Determine whether an append-only or guided continuation workflow is technically safe, and define completion as one supported reporting experience without accidental duplicate sites.