I have outdoor lights powered by an in-wall timer. Can I add a dimmer to work with both lights and in-wall timer?
How hard it would be top install will depend on the exact wiring but I don't see why you couldn't wire in a dumb (non-smart) dimmer after the switch on the switched hot wire to the outdoor lights.
This depends on the wiring in the switch box but if the switched side of the timer goes to two wires, one to the indoor light and one to the outside lights, it should be doable. However, if there is only one wire going to all of the lights, you will have to pull new wiring.
Make sure to use a dumb dimmer, just a slide or rotary control, and make sure it's rated for LED loads. You'll need to make room for the dimmer, either combine other switches or add another box or something. Disconnect the wire from the load side of the timer that goes to the outside lights, leaving the indoor lights connected. In its place, connect the line side of the dimmer. Connect the load side of the dimmer to the wiring going to the outdoor lights.
Note: I have no idea if wiring a dimmer after the timer is allowed by code but I don't see any reason it would cause a safety issue.