I made a keymap which I use for appending. It yanks to the 'x''0' register which is actually the default for yank and then appends it to your '+' register.
vnoremap <leader>y <Cmd>call AppendYank()<CR>
function! AppendYank()
normal! "xy"0y
call setreg('+', getreg('+') . getreg('x''0'), getregtype('+'))
endfunction