You know that for Parent = self you have Level = 0 and when you copying paths from ancestor, you're just increasing Level by 1:
create procedure USP_OrganizationUnitHierarchy_AddChild
@ParentId UNIQUEIDENTIFIER,
@NewChildId UNIQUEIDENTIFIER
AS
BEGIN
INSERT INTO [OrganizationUnitHierarchy]
(
[AncestorId],
[DescendantId],
[Level]
)
SELECT [AncestorId], @NewChildId, [Level] + 1
FROM [OrganizationUnitHierarchy]
WHERE [DescendantId] = @ParentId
UNION ALL
SELECT @NewChildId, @NewChildId, 0
END