Two things come to.mind, one would be to repeat the hammer and pull on each comvo so for finger 1 and 2 hammer flicl hammer flick next combo. Add more cycles as you want h f h f h f...
Second - and this fundementally changes the nature of the excercise into something different. Trill (hammer flick repeat) as HARD and FAST (as fast as you can without getting sloppy, you want the timing to be even and hammer and flick volume to be consistent) as you can for a set duration... 5, 10, 20 seconds.. for each combo. Probably don't need to run it across every string each time. Pick a string and fret and do it in that spot. Change strings and locations. Shoot for strong, even,, and increase speed. Be careful don't over do it, you can hurt youself. You are looking to get back to about the same level of tiredness you got when you first started. Like I said this goes beyond the intentp of the original exercise. Like graduating from regular push ups to clap push ups.
Be careful.
Shadow