Arkose: You can assign joystick inputs to keyboard commands using the DOSBox Mapper. Privateer disables a couple of keys when in joystick mode but most remain available. The DOSBox joysticktype determines how many buttons/axes the Mapper can see.

There is no way to have an absolute throttle working properly in Privateer. It only supports two axes and two buttons, and its throttle function (moving the Y axis while holding button 2) expects the relative input of a self-centring stick axis, not the absolute, non-centring input of a dedicated throttle.

The closest you could get would be to have the throttle work as two buttons with the centre being neutral, so pushing it up past the centre would send a constant speed up command until you return it to the centre, etc.

This same limitation applies to other games that lack a throttle axis. A few old games support sending absolute throttle percentages from the keyboard (so there are ways of adapting that to a physical throttle) but they would generally support a native throttle axis anyway.