There isn't enough information contained in a fasta file for converting to SFF. SFFs are flowgram files that contain information on sequence, quality and signal strengths. Why do you want to convert fasta to SFF? Newbler and Mira will both take in fasta files if that's what you want to use it for.
You can find information on SFF files here: http://contig.wordpress.com/2010/10/28/newbler-input-i-the-sff-file/
It is also a great blog for everything you want to know about how Newbler works.