It is not possible. Local pickup is primarly designed for merchant-managed locations. This means that the feature is typically used for locations that the merchant directly manages and controls. For third-party fulfillment services, local pickup is not inherently supported as these locations are not managed by the merchant themselves.
We are now also having this issue. We want to offer pickup at the warehouse (our clients are asking for it) but seemingly can’t find a way to enable local-pickup for a fulfillment service location.