You can't. That seems to be a feature they were going to add but never quite finished. Just like the triggers.

The only way to change the resource asteroids is to change the planets themselves with a mod.

That is correct.

I should be more clear. You can't get a mod to let you change the amount of resource asteroids in galaxy forge (at least easily). Rather mods can say, set it so all volcanic planets have 4 metal extractors, every single time. Or only two. Or have between 2 and 6. Its the planet type that determines the resource asteroid, the map just says where the planet is, it has no control over the resource asteroids.

No, its completely impossible to do it via a map. The game simply does not look at the map file to determine resource asteroids. It looks at the planet file. That's why you have to get/make a mod that changes planets, not maps. Its simply not programmed to do that at the moment. All you can really do is ask Stardock and hope they update the game to allow you to do this, which it seems they wanted to do at some point.

It may in the future if player interest in and support for the game stays high.  Asteroid presets, online map sharing and map triggers were all originally part of the concept.

Many changes to Diplomacy happened over a two year period including some huge bug fixes and engine and graphics improvements at the end of that time--when you would have least expected it.

The devs haven't shared how far they got towards developing resource asteroids, triggers and map sharing but if they made enough progress it's conceivable they may be inclined to add it in the future.

Sins has had a history of incremental but long-term changes.

I heard someone was actually able to use galaxy forge to do this but I haven't tried it myself.