Press RMB-->Select Nets. Click on desired net (GND, VCC...)-->RMB-->Add via. That's all. After this you may select this via and to copy it (Ctrl+C). Enjoy with PADS!
That was a bit too easy
You can also explore Via Shielding/Stitching capabilities of PADS. In Options > Via Grid you can find controls that govern via additions automation. Once you set them up, you can select Copper Pour or Split Plane geometry and Via Stitch entire shape from RMB (right mouse button). Via Shield will let you select one or two traces and add vias around them.