Hi,
(TL;DR at the end)
If you select all the clips you want to have a transition between them, you can then use the "Transitions->Fades->Fade through white" from the top menu, and VideoPad will then ask if you want to apply the transition to all the selected clips.
To change the duration for all of the transitions, BEFORE setting the transition you can go to "Tools->Options...->Editing<tab>" and set the "Default transition duration (seconds):" value there. All the transitions you add to clips using this method will try to have this duration.
To select multiple track clips, select one, then either
shift+click on another clip, and it will select all clips between the first and second one (including them)
ctrl+click to select clips one by one (this method allows you to select non-consecutive clips)
Unfortunately, if you want the transition to have differing durations, you'll need to edit the transitions individually.
TL;DR version:
Go to Tools -> Options... -> Editing (the tab) and set transition duration there
Select clips you want to put a transition on (the transition will go on the end of the clip
Click Transitions -> Fades -> Fade through white
Select "Apply To All".