In general there's no problem with adding a different oil-based finish on top of wood that was previously oiled. And despite widespread belief that this is necessary you don't have to allow the oil in the wood to 'dry' (cure) first, the two finishes should effectively meld together just as they would to a previous coat of the same finish.
You may find it beneficial to gently scuff or lightly sand before applying the "hard wax oil" but it may work perfectly well applied directly to the surface you already have.
If you're seeking superior waterproofing than linseed oil provides by itself I would strongly recommend doing a test of some kind to check just what level of protection the other finish will actually provide. Consumer usage reports and independent testing have begun to show that manufacturer claims for the robustness and water-resistance of "hard wax oil" products can a little 'generous' (read: exaggerated) compared to previously established finishes on the market, chief among them being plain old varnish.