I wouldn't put compost in there, as potting soil is very different from garden soil/dirt. I think you'd have problems with drainage from it.

Instead, why not get some fertilizer? For my peppers, eggplants and tomatoes I use the Miracle Grow tomato fertilizer, something like 18-18-21. It's grainy stuff (the size of raw brown sugar), I just drop a spoon of it into my watering can and fill with water; I noticed a huge difference when I started using it.