I have the Arduino UNO board and I want to know if it is possible to use both the USB connection and the external jack to power supply at same time
Yes. The Arduino will disconnect from USB power when the barrel connector voltage goes above about 6.8V.
But why would you want to use the "barrel jack"?
Please don't think to draw more than 200 mA total through the Arduino pins and that for only a short time.
If you want to power things that need more current (or volts) then use the Arduino to control transistors that will switch ON and OFF power that does not go through the Arduino, but power from a supply wired directly to the transistors.
Yes you can tap the VIN pin for more than 200 mA 5V but it's cleaner to run external power and ground lines.
Paul__B:
But why would you want to use the "barrel jack"?
Because, unlike the Vin pin, it has a reverse polarity protection diode?