Can a knee injury/weight gain lead to a metatarsal stress fracture?

Stress fracture. A knee injury or weight gain alone won't cause a metatarsal stress fracture. What they will do is put added stress on the foot/feet. Overtime, if nothing changes, stress fractures can develop.
Possible. Possible that knee pain and wt gain could cause abnormal gait and stress on feet which over time can lead to stress fracture of metatarsal. See podiatrist for evaluation and x/rays.