I believe you can start consider Using merge,
assume that you have two Google Cloud projects, Project-A and Project-B, and that you want to create a multi-project Workspace for these projects. Also assume that these projects are host projects for a Workspace. That is, a Workspace has been created for Project-A, and a different Workspace has been created for Project-B.
Because both projects are host projects for a Workspace, to have both projects in the same Workspace, you must merge the Workspaces:
In the Monitoring navigation pane, use the Workspace selector to select Project-A. This is the Workspace that is to be retained.
In the Monitoring navigation pane, select settings Settings.
In the Settings window, select Merge, and then select the Workspace named Project-B. To save the change, click Merge.
After the merge operation completes, the Settings window lists Project-A and Project-B as members of the Workspace. In this case, Project-A is the host project.
Also consider to check Multi-project Workspace best practice
1.- Create a Google Cloud project to be the Workspace host project. This project should be empty of resources. That is, it doesn't have any VM instances, Cloud Spanner tables, or other resources.
2.-Create a Workspace for the new project. For details, see Creating a Workspace.
3.- Add Google Cloud projects or AWS accounts to the Workspace by following the steps in Adding monitored projects.
By using this approach, you can move a project that was added in the last step, to a different Workspace, or you can remove it from the Workspace. The move and remove actions affect only the project being moved or removed. No other project in the Workspace is affected.
in this part of the public documentation covers them, mainly consisting of using a new empty GCP project as the host one for the Monitoring Workspace, with the resources to be monitored only being in the other projects.