How to combine different time-columns

Hey guys!

As you can see in the picture, I am working on a big table, containing parameters in the columns and observations in the rows and each parameter has its own time-column. My goal now is to combine all time-columns in order to work with the data. I already tried the stack-function, but unfortunately it fills up the gaps with the previous values which destroys the dataset. Filling up with NaNs instead would be perfect.

Do you have any tips how to manage that problem?

Kind regards, Spacey