Adding gears to adult tricycle
Probably the simplest is to replace the hubs with internally geared hubs.  There are many of those on the market with a wide range of gear capabilities.  You may find it easier to buy complete new wheels.  You need to check all the interfaces of the wheels-the width and diameter of the axle, how it mounts to the frame, the width of the rims if you are using rim brakes, the width and height of the tire fitting in the frame.  You need to add a control for the gears and cable back to the hub to carry the information.  Presumably your dropouts are horizontal so you can set the chain tension properly.  This will be maintained with internally geared hubs.  You might need to change the size of the front or rear gears depending on the gear range you desire and the hub you select.  That will impact the length of the chain, which is easy to adjust.  I would go into one (or a few) local shops with your question and see what they suggest.  It would be good to determine where your problem lies.  Do you want a lower gear for climbing hills?  Are you close, but just need one a little lower?  Are there some hills that are far too hard?  Do you need a higher gear because you can't pedal fast enough?  Again, are you close or far from what you want?  These will help the shop with recommending the proper solution.