If you're afraid of that, simply tell them you have been thinking of doing some modifications to your car and were wondering what their policy is on covering aftermarket products installed on the car. That way you can get your answers without disclosing what you already have.
I have full coverage on my car, our agent has just said as long as I have pictures to document mods to my car, and can provide a receipt or a price quote from a website or whatever stating what I paid (or the value) of the modification, that it will be covered. I don't have to disclose every mod I've done to my insurance company, I just have to have it documented that it was actually on the car in case it ever comes up. They covered me in the past when I hit a raccoon and tore off the complete front end of my body kit back in the day.
But, its going to depend on your insurance company, and even in the same insurance company I'm sure it could still boil down to your agent and how strictly they follow the rules
