Adding a buff with a trigger

There is no add buff command in the trigger list, only remove buff. So is there a way to do it with a trigger?

Buffs cannot be directly added through triggers. But you can add something like the Tornado slow ability (with no effect) which can provide a buff.

Just use an ability to apply a buff.
Unit - Add Tornado Slow to (Picked Unit)

The ability should do nothing but apply a buff. As blu said use tornado slow, or an aura that only effects self.

I tried that, but the buff needs to have a 20 second timer on it. Now I could do this with the wait command, but this needs to be multi-instable.

Then you can use a spell that does nothing. For example, make a Slow ability which doesn't actually slow at all, just lasts 20 seconds and adds the buff you want. That would be an easy way. You could also use timers/gamecache but it sounds like you don't know JASS so the best way would be what I said first.