@Thomas Moeller You can leverage the send-request policy to make any kind of requests, in your case fetch the token. This OAuth 2.0 sample (I suppose you are referring to this too), the same should strategy to work with just updating the request being made to match what NetSuite expects (their documentation might include the details).
Apart from this, there is no built-in policy that can leverage OAuth 1.0 (or even 2.0, hence the policy sample).