Hello Vasileios,
I would suggest to use the Initialization tab in the Simulation Parameters. Convert your values into two Octave arrays in a way the 1D Look-Up Table block can use it:
Time | Power
0.0 | 10
0.1 | 10
0.1 | 20
1 | 20
1 | 15
3 | 15
With this method you can output arbitrary waveforms using the modulo function.
See attached minimum example.