1. Can Shields be overcharged? As an example the ship's shield are at full and equal all around but the Capt wants the forward shield given 5 more shield points from diverted Aux Power can he do this?
Page 302 of the CRB details Shields. In that section, it states that at the start of combat, Total SP (Shield Points) generated by the ship's shields system are divided up into four quadrants with no quadrant having less then 10% of the total. Once combat starts, each quadrant has to have at least 10% of the available SP, which could be less than their starting SP, if the science officer does a Balance action.
An Engineer may divert power to shields and add up to 5% of PCU to any shield facing of the Engineer's choice. The Engineer cannot add more SP than the ship originally started with.
Provided that 10% of the total current SP are in each quadrant, a specific quadrant and go above the beginning value.
To answer your specific question, no.
2, What is the PCU cost for sensors? I can't find any in the rulebook.
Sensors are page 301 in the lower right corner. Very small section! The cost depends on how good the Sensor is.