Can treadmill running alone increase running stamina?

Yes. Treadmill Running and Road Running have some differences, but in many instances, they are the same.

Road Running, would provide more variations in terms of where you can run. Treadmill would be quite monotonous for obvious reasons.

On the Treadmill, it would be easier to program for different running surfaces…such as plain running, hill running with differing slopes (difficulty levels) etc. For Running Outside, it would take effort to find different running surfaces and also then, need to get to these differing locations.

During Extreme Weather Conditions, such as too much heat, too cold and such, it is advantageous to use the Treadmill.

Other than these, it shouldn’t make a difference if you are running outside or are using the treadmill.

In both cases, a good running (training) program, combined with proper Nutrition, proper Rest/Recovery and a good Strength Training Program is Essential to make good (Injury Free) progress.