Hi Alan. I live in southcentral Pennsylvania, USA. I am in the heart of the dairy cow region. You would think we have nothing in common, but as gardeners I think we do! I do not know about worm casts, but I can tell you about cow manure and the two may be similar.
US Farmers have found that mixing FRESH cow manure with water and letting it stand to ferment in a big vat makes the manure MORE beneficial than simply applying fresh or composted (old, dried out) cow manure to their fields. This "liquid manure" is very high in nutrients and micro-organisms.
Since worm casts are the waste product of worms, just as cow manure is the waste product of cows, I would suggest that you try using FRESH casts, mixed with water, and allowed to stand. Manure/casts that lie exposed to sunlight, rain, etc. will have nutrients leach away. More importantly, I would think that micro-organisms in the manure/casts will be killed off by sunlight, drying. The micro-organisms are as important as the nutrients.
I don't know about adding fresh casts to existing "soup." I don't see why it would not work as long as you added fresh water along with the casts so the mix did not become too strong.
