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Ah, make them trade receivers. Sure, that'd  be easy.

Depends on what you mean by resources, I've seen people use them a few different ways. For any case though, you can add them through any old data file. There's nothing special about the name of the data files.

For example: if I wanted to add gold, I could just make a new file called "gold.xml":

<game_data>
    <item name="Gold" icon="i_gold">
        <description>It's shiny!</description>
    </item>
</game_data>

That's all you need if you just want a new item that can be produced and used.

Adding a natural resource that can be found and exploited (like gold ore) is a little more involved, first add that resource through adding a resource tag, like so:

<game_data>
    <item name="Gold" icon="i_gold">
        <description>It's shiny!</description>
    </item>
    <resource name="Gold"></resource>
</game_data>

I would then have to add terrain containing that resource to the world, which is where mutators would come in. First, define a biome and terrain to contain the gold, which could be modified from the way tin works.

<game_data>
    <item name="Gold" icon="i_gold">
        <description>It's shiny!</description>
    </item>
    <resource name="Gold"></resource>
    
    <biome id="highlands_gold" name="Highlands">
        <terrain_mapping id="peak_small" likelyhood="2"></terrain_mapping>
        <terrain_mapping id="trees_sparse_0" likelyhood="1"></terrain_mapping>
        <terrain_mapping id="trees_sparse_1" likelyhood="1"></terrain_mapping>
        <terrain_mapping id="grass" likelyhood="12"></terrain_mapping>
        <terrain_mapping id="ore_gold" likelyhood="1"></terrain_mapping>
    </biome>
    <terrain id="ore_gold" map_image="map_tin_gold" is_walkable="true" is_swimmable="false" move_cost="1" resource="Gold">
        <base_animation type="idle" image="t_tin_gold" frame_width="64" frame_height="64" frame_count="1" facing="none" sound="none" layer="0.0" loop_time="0.5"></base_animation>
    </terrain>
</game_data>

Now the only thin that's left is to make that biome available during world generation. We can do this in the same file, or a new file to keep things clean.

<game_data>
    <add_element parent_type="region_feature" parent_id_attribute="id" parent_id_value="basic_landmass">
        <region_feature id="basic_landmass_highlands_with_gold" regions_per_occurrence="60" minimum_occurrences="1" shape="blob" min_size="1" max_size="1" margin="0" core_biome="highlands" fringe_biome="highlands">
                <inclusion_feature id="basic_landmass_highlands_gold" cells_per_occurrence="2" minimum_occurrences="1" shape="blob" core_biome="highlands_gold" fringe_biome="highlands_gold"></inclusion_feature>
            </region_feature>
    </add_element>
</game_data>

And now there should be gold scattered throughout the world