+ and # do the same thing in Music Markup Language (both are sharp) "-" is the only flat symbol I know of.
There is no method of direct translation for the key signing on sheetmusic.
Try to have a good day anyway.
You might want to change the key of the whole piece once you have it written out and exported as midi. The following place has midi tools to change keys (and many other functions) if you need to remove sharps/flats to make a song fit certain composing limits.
http://www.skytopia.com/software/midi/