I see three options:
Magnets or Washers
You could glue together some washers or magnets in 1.25 pound (or half-pound, or whatever) increments. This could change your jumps from five pounds per dumbbell to two and a half pounds per dumbbell (or even less). This is the best method to directly replicate barbells.
Some dumbbells are like miniature barbells, with the ability to put plates on the end. This would put you in the same position as someone with a barbell, though you do have four "ends" to load instead of just two.
Forget Microloading and Use Other Methods to Increase Strength
The point is not to microload. Microloading is a method to get stronger by adding manageable increments of resistance. Since dumbbells are an inherently different tool from barbells, we might want to use a method more appropriate to this tool to achieve our goal of getting stronger by adding manageable resistance.
The method I like is to wave-load the number of reps I do with a constant weight before a jump: three reps of 35 pound one-arm overhead press, then five in the next workout, then eight in the next workout, that turns out to stay challenging for three workouts, then ten reps in the workout after that, then I start doing three reps with the 50 pounder, then five reps in the next workout, and so on. The number of sets can remain constant or can be manipulated control total volume. I usually keep the number of sets constant.
I find that manipulating the number of reps is good for upper-body work, since my upper body seems to do better with, and even sometimes require, a higher number of reps. My squat and deadlift do better with microloading.