add (mix option) to mix Oxygen to Hydrogen Thrusters for bigger power

It doesn't. Nothing burns without oxygen, whether it's in space or anywhere else (space just has no innate oxygen, unlike Earth's atmosphere, but Earth's atmosphere has too little hydrogen to allow for powerful hydrogen-based thrust). Strictly speaking the "Hydrogen" tanks in the game already have to contain both oxygen and hydrogen in order to function, which also explains why the generators can only produce oxygen or "hydrogen" (since the latter must be both elements for it to be useful).

It also helps to think of the hydrogen tanks as being a combination tank with a shared bulkhead. You don't store a fuel-level oxyhydrogen mix in a single tank unless you want to have a spontaneously combustive time.

The ideal (since it encourages a more "engineering" approach) would be to separate the two and correctly allow the Hydrogen Thrusters to require bi-propellants. But this would also need to include new tanks (since now you'll need both to run a thruster) complicating things somewhat, and render many, if not all current hydrogen-based designs inoperable. Barring the use of an oxygen-poor ratio (less than 6:1 O:H), this would also leave you in a situation where introducing additional oxygen would actually decrease the amount of thrust - not increase it.