You can put your flat strings into any software and have them increase and decrease the sounds gradually at any bar. It does not have to be midi based, however you can render a flat strings track and adjust to suit your needs.
On the other hand, you too can spend days learning how to program in midi, adding velocities and other exciting stuff. Very useful. Just be forewarned that about the only thing new in midi these days seems to be dying a very quick death.