When you hit your power up, you should just add whatever amount of you want to your timer.
So, say, in your Player Pawn (or controller, or whatever), you have your main timer running. When you hit a power up, you read the timer value and add the power up to it, and the counter continues to run.
I did this very quickly:
I added a timer to the player character:
And I made a quick Actor BP with this on it:
So, when my character overlaps the BP, it pulls the timer value from the player character, adds the power up timer value to the timer and sets the timer value.
Hope that helps!