i have Pentium(R) Dual-Core CPU
it's 2.7Ghz x2 can i add another cores? to make it like 2.7Ghz x4?
It doesn't work that way. Upgrading a CPU means removing the old one and slotting in a new one. If you are referring to core unlocking, it is only possible on some AMD Athlon/Phenom chips.
What is the make and model of you motherboard?