I don't remember seeing commercial compost as powder or pellets but I might be displaying my ignorance
. Compost is decomposed plant material so the few times I've bought it here it has looked like...compost! When I've used it generally it has been from our own livestock so not purchased. Are you thinking of fertilizer, which does come in powder or granules?
You can add compost any time. I would tend to do it when the soil is a bit warmer and the worms/microorganisms are more active.