Yes. You can add workstreams to your existing project and schedule future hours against the workstreams.

If you have past assigments that are NOT linked to any timesheets, you can link them to the workstreams by clicking on Change Workstreams

However, if you have existing timesheets/ actual hours linked to the assignments of your projects, you cannot add them to the workstreams.

The Change Workstream function currently only supports assignments that are not linked to timesheets.