Not readily.. IMO you'd have to replace the entire controller circuit...
You can't just splice a dimmer in between the DC supply and the fixture... Dimmers work by switching the power on and off really fast, and changing the length of time that it is "on" each time. Putting a dimmer between the light and its power supply would end up cutting power on-and-off to the internal controller that's trying to run 24/7 mode, not just the light.
For this reason, generally speaking when it comes to dimmers, you should only have one. You can't dim a lamp that already has a dimmer and expect reasonable behavior..
I've tried to point out many times this light has 2 uses:
24/7 mode - not adjustable, but cool. Probably too much light for low tech in tanks under 20" deep.
dimmable light - using an external timer. You get flexibility, but loose the 24/7 mode feature.
Given the price point, it's not too bad that you have to choose between one and the other.. remember this sells on Amazon for only a few bucks over the price of a planted+, and that just has 2 on/off switches..
I also don't think the internal controller can be readily modified... see my review which has some pictures of the internals, and notes about the trickiness of taking it apart:
Finnex 24/7 full review.