To configure a Jira project with existing tickets for use with Ketryx, perform the following in the Jira project settings:

Under Issues / Types, click Actions / Use a different scheme and switch to the Ketryx issue type scheme (or a desired variant with a subset of the issue types). You might have to define mappings from your current issue types to the Ketryx issue types.
Under Issues / Screens, click Actions / Use a different scheme and switch to the Ketryx issue type screen scheme.
Under Workflows, click Switch Scheme and switch to the Ketryx workflow scheme. You might have to define mappings from your current workflow statuses to the statuses used by Ketryx workflows.
Note that this only works for company-managed Jira projects.