Apart from the external RF power settings there isn't a lot you can do inside the radio that wouldn't require extensive electronic 'surgery'.
There are three service jumpers on the PA stage of the 6900 but these are to disable the PA altogether and two jumpers for setting the biasing to each of the final MOSFETs as they are run in a parallel pair. (Not a push-pull as some suggest!)
You could maybe run a tap from the first PA MOSFET but you would also have to match your impedance with additional circuitry as well as provide an additional low pass filter arrangement. Again, not simple.
The simplest answer is an external attenuator or RF PAD but you'll have to make sure it's more than powerful enough to handle your power dropping requirements. These can be expensive. You could of course build one using a 'Pi' or 'T' resistor networks but you'll need high power resistors that are inductance free such as RF carbon composites and again these we expensive.
There are RF attenuators available on eBay at cheap prices, these are usually a board populated with surface mount resistors. They are low power devices, (<10W), so not much help. I've seen plenty of those 'beauties' with the board burned out!
So sorry mate, not really any simple solutions I can think of. My own HF set has a linear switch and separate RF output for things like external linears or transvertors but it's not something usually found on 'CB' type radios.
All the best,