It is very easy. You can set the behaviuor of AE-L/AF-L button in the f7 (Assign AE-L/AF-L button) custom settings menu.
For your purpose select f7 and then "AE-L/AF-L button press" submenu. In the submenu select "AE lock (Hold)" or "AE lock (Reset on release)" setting. Both lock the AE value when AE-L/AF-L button is being pressed, but the first setting holds the value until you press AE-L/AF-L again, while the second one holds the value until you press shutter release or until the meter switches-off or you press AE-L/AF-L again. Very useful seting, I use it all the time.