It's really going to be dependent on what you want your final format to be in, Solidworks Drawing file, PDF, etc.
In my workflow we convert all Solidworks Drawing files to PDF for final release so I would simply import a completed PDF without the rocks into Adobe Illustrator and draw the rocks and gravel and create a new PDF. This would give me a good combination of technical and visual information needed.
There are several other ways I could think to do it also depending on required outcomes as said before, but all of them would require a technical illustration software IMO. If you don't have one you can use Inkscape which is a free software.
I would be inclined to use a vector based program such as Illustrator or CorelDraw. Export the SW drawing as PDF. Open PDF in said program. Copy the central element and paste into a new layer which is on top of the original. Create a new layer which sits in the middle of the other 2. Create a few objects of different form and fill colours, then copy/paste them randomly. Save-as pdf.
In AutoCAD, the hatch was called riprap (or something like that, which is what it's called in road/site construction.
There's also gravel hatch which is in DraftSight.
If you can export your sketch into DraftSight, hatch it there, explode the hatch and then bring everything back into SW.
Downside: all links are broken.
You may not need to explode the hatch back to its bits. Try bringing it into SW as a hatch directly.
There may be a way to decipher DS hatches to transform them into SW ones, but I don't know how.