Before investing in a CO2 system I assume you've checked other things.
If plants are not doing well, first thing to think of is always light. If your light is okay for the type of plants you are trying to grow, then CO2 is normally the next limiting factor. If your light is wrong, CO2 will make very little difference.
With the right light, the right CO2, and a suitable water chemistry, most plants will grow. The substrate is in my experience, the least important factor, I used to use gravel and had good plants. I now use laterite under swimming pool filter sand and have great plants.
I have tried under tank heat pads and in substrate Dennerle heater cables and have never found they made the slightest difference.
You cannot add laterite to an established tank, at least not easily, it will just form a suspension in the water. You may be able to form a slurry/paste and devise some means of injecting it through your substrate, but it sounds like more trouble then it's worth.