There is no released tool to build fiasco images, but you can backup and flash each piece separately.
Just don't use dd, mtd flash memory is not safe for dd (any bad block would interrupt the dump leaving you with a broken image), use 0xFFFF or mkfs.jffs2 to build an image.
There are some threads here about how to use mkfs.jffs2 and the dualboot scripts are a good example.
I have attached here a build of 0xFFFF for the tablets, since you won't find one in its home page.