I've done this on a 407.
If the 407 has the same rigid tank then it'll be the same, but I seem to remember something about a bag?
The claim is that the tank is three litres. There is no level sensor, just a counter in the ECU.
I found that the tank is dosed with about 13ml per 100 litres of fuel.
I found that the warning came on when the counter reached 700ml -- so plenty left.
After resetting (after refilling) the counter went to 3200ml.
On the 407 the connector for the tank is visible from the rear of the rear passenger side wheel. You can get to it without even jacking the car up let alone removing the wheel.
The colour of the connector is supposed to tell you what type of additive it needs. There are diferent types. Don't get the wrong one!
This is probably what you need: https://www.spareshq.co.uk/product/additives-b1050/pat-dpf-fluid-1lt-COLPAT1L
They have a 5L drum too. This is the cheapest place I found.
Using the refilling kit is a bit messy, so put some cardboard down. And don't get it on you -- wear gloves.
In principle you undo the pipe from the tank and connect the pipe to the feeder bottle, then fill the feeder bottle and hold it up while the new additive runs down into the tank.
In practice you need to squeeze the feeder bottle to get the additive to move. Then, holding in squeezed in, turn it upright and loosen the top to let it expand without sucking the additive back from the tank. Then top up the feeder bottle and repeat.
Afterwards you need to reset the counter in the computer.
Now: suppose that originally it was full, that the counter is on 600, and you put in 2000 and it resets to 3200.
Then there's actually only 2600 in it so when the warning next comes on at 700 there'll actually be 100 left.
Therefore you might get away with putting only two litres in.