If you have some way of manually circulating the water, such as using a pool with a brush or net at the end you can use for stirring the water in the area where you pour, then that's better than nothing. However, it really is better to pour slowly over a return flow with the pump running. Muriatic Acid is VERY strong so until it gets diluted it can cause damage. Now that said, it would be far worse to pour in the acid without the pump running in a vinyl pool since vinyl is particularly sensitive to low pH. A plaster pool such as yours will be heartier in that regard.
Do you not have a way to manually turn on your pump temporarily?
Richard