If you want to have a custom exposure after the timer goes off, you can use the Timed Release and Self Timer mode. This mode combines the Self Timer with the capability of keeping the shutter open for a certain time. It has two parameters: Timer and Exposure.
Timer: This parameter defines when to trigger the camera.
Exposure: This parameter defines how long to keep the shutter open.
Once you set the parameters, you will see to counters at the bottom of the screen. The counter on the left displays the timer. It will count down from the timer value and when it goes off, the second counter will start showing how long the shutter has been kept open. The shutter will close when the custom exposure value is reached. The camera must be in BULB mode to have flexible exposure.