On the Help Guide, these are the Permission Levels that could be set on a Role for a record:
CREATE - User can create new and view existing files.
EDIT - User has access to create new, view existing, and edit existing files. The user cannot delete existing files.
FULL - User has access to create new files and view, edit, and delete existing files.
Thus, having the EDIT only mode is still an Enhancement and this is filed under Issue# 80667- " Ability to have the "edit" permission without having the create permission on a record within NetSuite"
This can be done via script by restricting a User's action on beforeLoad of via Workflow.
Here are the steps to achieve this:
1. Navigate to Customization > Workflow > Workflows > New
2. Enter the following fields:
a. Record Type: (Record you want to restrict)
3. Click Save.
b. Sub Type : (Record you want to restrict)
c. Release Status : Released
d. Event Based: On Create
e. Trigger Type : Before Record Load
a. Trigger On : Entryb. Event Type : Createc. Condition : Not User Role = Administratord. Parameter Text: Action not allowed', 'Your role is not permitted to Add this type of Record.