render out your frames in maya.., in Fusion, drop a 'loader node' into Fusion flow work space, a 'background node' and a 'merge node'

put the maya files into the loader node and your background, a back plate into the background node, to out put.., its 'output' in Shake, maybe its the 'saver node' in Fusion?? i think thats how its done. a straight composite!? watch your alpha though, there might be a gotcha there, cant remember but it rings a bell.

remeber to set your settings (frame rate, PAL or NTSC, ect) and save the Fusion file before importing or doing anything in fusion,

im very ignorant of Fusion so i cant really be sure of the above but it wont take you long to check.