Can a very large subserosal uterine fibroid cause dvt?

Yes. A large uterine fibroid can compress an iliac vein and cause dvt. It is fairly unusual, but certainly possible.
Unlikely, but... That scenario would be very unusual, but if the fibroid happened to be very large and push on a deep pelvic vein (iliac), it could cause stagnant flow within the vein. Stagnant venous flow is a risk factor for dvt. Diagnosis would be via ultrasound with doppler and/or ct of the pelvis with contrast.