Hi
I don´t have sure if that will work. But You could sculpt using any 3d sculpt program around (mudbox, zbrush, modo, etc). Make your model X, change the level subdivide and add some details, creat another level subdivide and add more details. Go to each level subdivide and subdivide again until you get same number of vertices and export OBJ file.
You need that all export files has same number of vertice. Go to XSI and make morph with them.
Excuse my english. Maybe I didn´t be clear enought, but I hope you have understood the process.
good look.