Hello 1975, welcome to the forum.
Take a look at this thread: http://nepacrossroads.com/viewtopic.php?t=1952
It discusses several options for hooking up a boiler into an exsisting system. As I stated in the thread, I'd go with a series hookup, tie into the cold water return manifold, route this through the coal boiler. then from the hot outlet of the coal boiler into the original cold water return in the oil boiler.
This way the coal heats the return water, keeps the oil boiler above it's set point [you would want to lower the oil temp setpoint]. If the coal fire laggs behind in very cold weather or the coal fire goes out, then the oil burner kicks in to add heat to the system.
There may be some standby losses of heat from heating the oil boiler and piping, but if the boiler is in your basement this just adds heat to the house.
Hope this helps, Greg L