the for loop is indeed not that big, but perhaps anyone has a "logical-only" solution.
in the meantime if anyone else comes across this, this for-loop does the trick for me:
rs='4'
for n=1:length(new_Numbers)
[i,j] = ismember(new_Numbers(n),cell2mat(Old_Array(:,1)));
if i
Old_Array{j,2} = [Old_Array{j,2} ',' num2str(rs)];
else
pos_new_entry = length(Old_Array)+1;
Old_Array{pos_new_entry,1} = new_Numbers(n);
Old_Array{pos_new_entry,2} = num2str(rs);
end;
end;