On this one, you really do need a good plumber to visit first. If you have a pressurized sealed system, then you cannot fit a soild fuel back boiler into that system. You can fit it to heat only domestic hot water, but you will need a separate independent coil in the cylinder for this and not share the oil boiler circuit one, so it will mean changing the cylinder and the back boiler output should be sized accordingly and you will have to allow for expansion for this circuit with a f & e tank in the attic and a vent pipe of course.
It your system is open vented, then of course you can tie in a solid fuel back boiler, however, there will be some requirements, such as you will need to bypass the zone valves fitted with non-return valves so that the oil boiler circuit is not bypassing them but allowing the bb to. You will also need a non-return valve on the back boiler, so that the oil boiler does not heat the back boiler. The back boiler will require its own circulating pump and pipe stat (located on the flow pipe close the bb) and independent of the circulating pump of the oil boiler. This pump should not work against the pump of the oil boiler, i.e. if oil boiler pump is on flow, then bb pump should be on return. Just make sure they are not pumping to eachother or pulling away from eachother.
I am most certainly not covering everything here, so please do get a good plumber to visit first as many safety issues MUST BE complied with and without inspection, it is too difficult to ascertain.