Anytime and all the time is the right time to add
compost from a known and trusted source. Finished compost (including some manure composts) can be added to the surface soil whenever you have it. Worms and microbes will take it from there. This should be considered an ongoing process, not just a starter exercise. As long as you provide food (compost) for your soil organisms, they'll eventually break it all down into usable food for your plants.
While you're waiting for your soil to become nutrient-rich from compost, worm activity and microbe interactions you could apply a processed seaweed or seaweed and fish emulsion combo foliar feed directly to your plants, as they need it.
I'd offer a word of caution about the use of fresh kelp, however; it might be best to compost it first so that some of the salts can be leached away beforehand.