You need to remove a litre of water from the system first before you add a litre of X400. (drain a little more to give youself some extra space).
That is of course assuming that your only putting one, 1 litre bottle in.
No need to shut system down if the radiator valves are holding.
Although if the system has been on, the water will be hot, hot, HOT.
Choose a radiator.
Close off both valves. Take a note of the number of turns on the lockshield valve, you will open this up the same amount later on to keep the system balanced (assuming it is balanced in the first place and that the other valve is a TRV).
Get a container and some rags, crack open the "nut" at the radiator. This would be the one at the bottom, between the lockshield (or TRV) and the radiator. The nut should only be loosened a little, not removed completely. Get the container under the nut. Be aware the liquid will not drip straight into the container in a nice neat controller manner, it could squirt out in a variety of directions.
Drain some liquid into the container, you shouldn't need to open the bleed valve at this point, try to avoid getting the liquid onto the carpet etc.
(it could leave a stain, especially if its sludge)
You will be able to retighten the "nut" to control the flow.
Once you have removed the amount of liquid tighten the nut back up.
This may require several visit depending upon the size of container and available space.
Remove the plug from the top of the radiator and use a suitable funnel (or other arrangement) to pour in your X400.
Replace the plug.
Open up your valve(s) by the correct amount (number of turns from earlier).
Once you have it running for a while you may want to bleed the rad to remove any air.
Job Done...........Nearly !
As an alternative you could try syphoning liquid out of the top plug if you have problems at the pipework.
There may be alternative ways depending upon the layout of your system.
Check to see if there is a readily accessible system drain, this could also be used to drain out the required amount, and will be needed later.....
This method ensures all chemicals are in the system, at the right place, to do the most good, straight away.
X400 can stay in the system for up to 1 month (if memory serves), after that ALL the liquid needs to be removed, (perhaps even consider a flush arrangement?) to ensure no X400 remains in the system. That will be where a readily accessible drain valve comes in, some systems may have more than one drain point. It should be drained from them all.
Once you have done all that I would consider using the same technique to put in the X100 (and X200) into the system as the expansion tank in the loft usually has the pipe coming from the side, not the bottom, resulting in some additives staying up in the tank.
Then the refilling of the system.........woohoo.