Yes, but only after the linseed oil is dry.
If it was raw linseed oil, it takes quite a long time (how long depends on temperature and humidity). If it had artificial driers added, it will be much faster.
Marine spar varnish is compatible with linseed oil because the main ingredient is linseed oil. This helps keep the dry/hardened finish very slightly flexible, so it doesn't chip or flake. The same is not true of urethane-based finished.
Be sure to use a formulation that includes UV protectant -which all marine spar varnishes will contain in varying amounts.