I've seen manufacturers a couple of times put suspension forks on gravel bikes instead of rigid carbon forks. Yes, this fork was not in the top price category from RochShox or Fox, but still, is it possible to replace a rigid fork on a gravel bike with a suspension one? I plan on getting a RockShox Reba fork with 100mm of travel. And no, I'm not going to ride trails or use gravel for downhill, it's just that in my area there are a huge number of bumps, holes, and the quality of the roads itself leaves much to be desired. (While maintaining the same wheel dimensions, including diameter, I can also reduce the travel of the fork by pumping less pressure into the fork to offset the difference in geometry of the previous fork that was in stock)
Swapping a fork (rigid/suspended, suspended/rigid or even rigid to another rigid) is subject to the following considerations. I would assume that the wheel diameter is not changing.
Geometry changes. Putting a longer fork (and suspension forks are generally, although not necessarily, longer) changes many bicycle dimensions, such as reach, headtube angle, bottom bracket height. This also changes the so called mechanical trail of the front. Unless your calculations show that the changes are minimal, you should be prepared for the fact that the feel of steering, and general frame fit, will change, and not necessarily for the better. To answer this question, find out and compare the values of A-C (axle to crown) and wheel offset of the two forks. You would need to compare these numbers even if you do a rigid/rigid swap.
Clearance issues. As a suspension fork compresses, the wheel comes closer to the down tube of the frame. Toe overlap may become real or worsen. While the toe overlap is undesirable and dangerous, the wheel touching the downtube is catastrophic. Also, suspension forks have wider crowns, which may touch the frame when the fork is rotated about 90°. You should experiment with the new fork at its maximum compression to see if the clearances are big enough.
Then there are generic issues of parts compatibility such as steerer dimensions, braking system, wheel axle length and diameter etc. I assume you've ensured that there will be no problems with them.
As for you suggestion:
I plan on getting a RockShox Reba fork with 100mm of travel.
Unless you can shorten its travel by opening up the fork and moving some spacers around (not a common feature for RockShox products), this fork will generally be too long. Lowering the air pressure will still keep it at 100 mm of dynamic travel. The increased static sag will only visually hide some of the travel but won't make it go away.
A small answer to the last part of your question.
I do not recommend trying to adjust the travel of a fork by reducing the air pressure, this will have a number of undesirable side effects.
If you run a pressure so low that you have ~50% sag you will find it blows through its travel and bottoms out on even moderate impacts. You will also find it bobs whenever you put power down which is inefficient. Most importantly, you will also introduce a phenomenon known as brake dive - when your weight shifts forward during braking, the fork will compress and alter the geometry of the bike. In extreme cases such as emergency braking, this could throw you over the handlebars.