How can I make an NLA strip longer to fit more keyframes? I made an animation and dropped it as an NLA strip - now I'd like to tweak that strip to add more keyframes and make the strip longer.

I thought that the Extend (E key) function does that - but no it only stretches the strip. I don't want to scale the strip, because then I'd have to scale down the keyframes to actually fit more of them in my strip - and I can easily break the animation timing while attempting this.

How can I make an NLA strip longer to fit the extra keyframes?
Open the NLA editor
Select the strip you want to make longer
Open the properties panel with the N key
Go to the Action Strip tab
Enable the Sync Length checkbox - this will make the strip longer if you tweak it, making the underlying action longer.