Your questions are still unclear as to exactly what you mean, however:
How would they connect to the existing radiators
What was the heating flow from the boiler is split, with two 2 port zone valves, one of which supplies the radiators, the other the hot water cylinder.
You will obviously need new controls such as a programmer, thermostat etc. to select hot water, heating or both.
And can’t they take a hot and cold water feed from the nearest point
Hot water is supplied from the hot water cylinder.
Cold is either direct from the mains, or via the control group on the cylinder if it is intended to supply things such as a mixer shower.
As it's a combi boiler, it would still be possible to use that for at least one hot water outlet independent of the new cylinder, but that would require separate plumbing connections to the boiler for whichever outlet is supplied.
As for where the pipes connect - in theory it is possible to connect to existing radiators and hot/cold outlets at any point, but that may lead to problems depending on the pipework.
Long lengths of pipework and those with excessive numbers of bends in will restrict the flow, as will smaller size pipes.
Some examples - you would not want the radiators all connected via a 15mm pipe, as the flow will almost certainly be insufficient for all of the radiators.
You would not want the hot water to travel through long lengths of pipe all over the house as it will increase the time taken for hot water to reach the outlets, and waste a significant amount of heat with the hot water left in the pipes every time a tap is used.
