Now in each signature we have to specify r and s explicitly. What about choosing some "shared K value" and calculate it as kValueA*kValueB*...*kValueZ*basePoint? In this way all parties can agree on the same r and safely exchange partial s values defined as "(z+r*privKeyA)/kValueA", "(z+r*privKeyB)/kValueB", etc. Then, everyone have to divide received value by its chosen part of K and in result everyone can receive sA="(z+r*privKeyA)/(kValueA*kValueB*...*kValueZ)", sB="(z+r*privKeyB)/(kValueA*kValueB*...*kValueZ)", ..., sZ and include it in the final transaction.