From vim you can run shell commands. So in this case I use:
:!touch somefile.txt
and then hit r to reload the nerdtree window.
The other thing to do is to just start the new file from within vim.
:e somefile.txt
One handy thing for this is that in my .vimrc I auto change the cwd to the directory my current file is in:
" Auto change the directory to the current file I'm working on
autocmd BufEnter * lcd %:p:h
This way if I'm editing a file and want another one in the same place the path is changed right there. Opening any file from NERDTree sets the directory to the one that file is in.