If you have an existing record type in Salesforce and that you want to use in Sweep, you must first import that record type to the Canvas as a Funnel. Once the record type has been imported to Sweep, you can manage stages, gates and automations for that Funnel.
In order to do so, see this video or follow the steps below:
Open your Funnel editor
Click the cloud icon with the arrow pointing downwards
Click the object of the Funnel that you would like to import
Choose the record type you would like to import
Choosing "Preview" will show you the various steps of said process
If there are multiple fields that could be your leading field for this Funnel, the preview step will allow you to toggle and select the appropriate field
Choosing "Add" will immediately add it to the Canvas
Drop the Funnel on the Canvas, and begin using it!