I hate to revive a dead thread, but I've seen this question thrown about a bit without resolution. Call me Charlie here, but I've been spending the last hour or two just looking at pictures and doing some research on this and being the EcoSport and 2008-19 Fiesta are built on the B2E platform, I believe it is reasonable to say that yes, you CAN bolt the rear suspension of an EcoSport to a Fiesta, HOWEVER!!!!!! The rear beam axle is also different in its spring pockets and hub locations relative to the torsion beam. It is possible the springs and shocks (again based on pictures alone) will bolt in, however, I would expect the result to be a significant rake from rear to front (as the hubs on the EcoSport rear beam appear to be located 2-3" above the beam whereas the fiesta's hub appear to be located centered on the beam) as well the the shape and depth of the spring pockets is slightly different. As such, I feel this may cause a bit of a funky look with a standard Fiesta beam essentially putting the rear end 2-3" higher than expected on the EcoSport springs and dampers and it is for this reason it may be necessary to replace the entire beam as well. Conversely, one may consider using the EcoSport rear beam on fiesta lowering springs/coilovers if they wanted to get the stupidlow look as it would raise the hubs relative to the beam and put the rear wheels further up into the wheel wells with all the rest staying the same.
Conclusion: Mounting locations for springs and dampers appear the same, likely will bolt on but may give significant rake due to different design of rear beam axle. Would recommend entire 2WD rear beam axle replacement for ideal rear suspension seating and performance.