Moles, freckles, birthmarks and other skin marks

The easiest way I know is to edit the texture in the paint program of your choice. You could also use decals to achieve the same effect...you'd still need to crack open a paint program to set the decal up.

I second (third?) the use of LIEs for this sort of thing. Remember, if the mole isn't exactly where you want it to be on the figure, you can use the LIE to add X- and Y-offsets to adjust the position of the overlay, and you can adjust its scale, color, opacity, perform masking, etc. all from the LIE editor.

If it's just a single mole you want to overlay, find an image of a mole (if it's more complex than a simple dot, that is) and put it on a transparent background. Use that image as an overlay layer in the LIE for the Base Color texture of the surface, then scale it to the size you want and adjust the X- and Y-offsets to get the mole where you want it, and adjust the opacity to make it look the way you want against the skin.

LIEs are very useful for things like this, and adding tattoos, decals, and other surface detail to figures and props--definitely a technique worth practicing.

@littlepleasures

I've never encountered any noticeable performance hit using LIEs, but then I'm using higher-end hardware with more resources to spare.

Looking at the way DAZ handles LIEs (at least in v4.21+), though, I don't see why their use would slow things down. When you apply an overlay with the LIE, DAZ creates a temp file that has the new layer(s) baked on the original texture, and the temp file gets used as the texture for that surface. The baking process takes place when layers are added or modified, so it should have no impact on render performance. As far as the rendering engine is concerned, it has a flattened texture file (the temp file) to work with, as usual.

What I appreciate about the LIE system is the non-destructive nature of it--it's the same sort of layer-based system that tools like Photoshop use, and once non-destructive editing habits get drummed into you as an artist it's hard to go back. The idea of editing a surface's original texture (rather than just applying a removable overlay) goes against the grain 😉

Which is not to say that you can't just make a copy of the original texture, give it a slightly different name, edit the copy, and create a new DUF file to point to it as a new texture variant, as if you were creating your own texture expansion for the product. That would seem to be the cleanest route if you want to avoid using LIEs, but it's more complicated, not facilitated by the GUI, and not portable to other figures or props.

As far as shader properties are concerned, you can also apply LIEs to other textures, not just the diffuse (Base Color) map. You could apply a LIE overlay to the bump (Normal) map, or any of the other textures if you felt the need. Those "scarification" products do this to make flat tattoos look raised with bump mapping, all with LIEs.

The rendering engine uses the pre-baked temp textures, so any effects you could achieve by editing the texture files directly can be achieved with LIE overlays.

Possibly your dim view of LIEs was formed by earlier implementations that were less than efficient and kludgier, but I haven't encountered any of the limitations you describe in 4.21, so DAZ may have fixed those issues in the interim.

Everthing mentioned above, and...
Skin Builder (and Brow Remover)
Sun-Kissed Facial Freckles for G3, G8, and G8.1

But or a specific spot, I usually go to the Surfaces tab, select my model, go to the appendage or part that needs the mole or whatever, under base color select the image and when the drop down appears of "Browse", "None", the other image locations, select "Layered Image Editor" and add a gif or pdf of a "dot" on a transparent layer (you can create this in basic image editors) and place that on your Arm, Face, left buttock, what-have-you. Using the editor, you should be able to place it where it needs to go.

Often times I'll take the entire texture and edit it in photoshop, as others have said, then use CrazyBump if I need a normal map.