Drives me nuts that my wizards and physicians have to go through two tedious steps over and over when collecting plants and minerals. Is it possible to make or is there already a mod to combine "discover" and "gather"?

I dunno whether such a mod already exists, but I can't see why it wouldn't be possible to create one.
I'm currently working on something else, but I can look into it for you afterwards if no one else answers.

If you discover and don't gather from the node it loses the discovery, but once discovered and herbed/mined it should never need discovered again for the rest of the kingdom.
Your "over and over" comment confused me, do you mean you're tired of doing that all again in new kingdoms?

In new Kingdoms, and when I haven't had time to have all my heroes look at all the herbs and minerals they are allowed to in any particular kingdom. It takes so long for them to discover something and then to gather it it's a pain to take two steps when one would do it.

Alright, so I found some time to take a look at it.
It's certainly possible to do this, but it would be a mess doing this with a simple scripting mod, as you would have to replace like everything related.
And that would probably introduce a whole lot of currently unknown issues.

I suggest that you PM Grim with a link to this thread and ask him nicely to implement this into his core mod.

The type Sims3.Gameplay.Objects.Spawners.WorldGatheringSpawner needs to be modified to use a combined version of two of its methods:

• SearchForNode (Discover)
• CollectObject (Gather)

I can help with the actual modification if he doesn't have the time to do it himself.

No need for a PM. I'm looking into it. Give me some time to fiddle with it.

May the Fourth be with you.

I stay off the computer for a day and someone comes along with a solution! Wow! Thank you!