Suggestions on making exterior fiberglass steps look rustic?

You can use paint to make them look like wood. You will need a special tool that rocks back and forth while you drag the paint. Do a test run until you're comfortable using this tool. I read that you should stick with paint made especially for fiberglass. If your fiberglass is an epoxy resin, you should use an epoxy-based paint. If your fiberglass is polyester-resin, you should use polyester-based paint. I hope you find this info helpful. Best of luck to you.

Fiberglass is a difficult material for paint. It has a clear coat on it that does not play nice with most other products. There is a tub&tile paint from Rustoleum that works on it. I'm not sure it will look like wood however

I would not recommend doing this, simply in the interests of safety. Painting or putting anything on the fibreglass may very well increase the slipperiness of the surface, resulting in it becoming a hard- to-maintain dangerous surface.