The game already does that.
A missile in the game has a nuclear warhead. It does not explode directly in contact with the target in space combat; it's a close proximity detonation that, due to the way the damage model works, might well be a Casaba howitzer style weapon projecting a jet of nuclear powered plasma at the target.
In a bombardment role the nuke does multiple things, it explodes on or near a target, doing blast damage, it stirs up a lot of dust and smoke, which affects how much sunlight the planet surface gets, and it creates a quantity of radioactive materials, which impact the planetary population's health. What you do when you tick the enhanced radiation warhead box is instruct your designers to prioritise the production of radioactive materials (and thus long term impact of the nuke) over the blast damage (and thus the short term impact). Nuclear bombs like this are called 'salted' nukes.
The reason you are unlikely to use standard high explosives in space is because the energy density of a fission bomb is three orders of magnitude greater than an explosive, a pure fusion bomb would even reach 4 orders of magnitude greater than an explosive. This means you can shove a whole lot more boom in a handier package, and in space, where propagating an explosion is difficult, the harsh radiation coming from a nuke works quite well.