I am pretty sure that there is an IsFalling or IsFlying (pure) function which returns if the pawn is falling or in air. When it is true you can just take the up vector. scale it up a little bit and update the velocity.
Edit:
Here is a small tutor how to do it:
Create a new variable Fall Speed. (It will be the rate that we will scale with)
Set it’s default value to -1200.0 (Negative value).
Then there is an Is Falling pure function which returns true if the character is falling false if its not.
The final blueprint should look something like this:
I hope it helps cheers! 