Stock Android doesn't have an option for long-pressing a number on the dialer to act as a quick dial. The easiest way for you would be to place a Direct Dial widget on the screen. Go to your Widgets screen, locate Direct Dial, long-press it and drag it onto a homescreen. It will then prompt you to select a contact to dial, and a specific phone number from that contact. You can then tap that widget for a one-touch dial.