Sounds like you are trying to migrate to Atlas.
In this case, the Atlas Live Migration Service is what you need.
You cannot add directly on-prem nodes in Atlas. This would be a security issue and how would Atlas administer / monitor / upgrade these nodes?
See my answer here:
If you want to replicate data, you could manually set up Change Streams and push the write operations from on-prem to Atlas, but this is a lot of work (need monitoring, restart algorithm, etc).
To keep it simple, I would just migrate the service in one shot when you are ready by starting from an empty cluster in Atlas equivalent (in size) to your current on-prem machines (or bigger) and I would migrate in one shot using the Migration tool.
Cheers,
Maxime.