You can work out what resistor value you'll need using V=IR and P=(I^2)R.
I presume you'll need a resistor for each bulb.
Sizing the resistor to make the new setup (i.e. the current drawn by the resistor together with the LEDs) use the same current as the old setup should fool the OBC, though you can probably get away with a higher value resistor and still fool the OBC.
Bear in mind that the resistor(s) will probably dissipate several watts (just like the old bulbs) and so might get hot. When you choose your resistors, you'l need to get one(s) with the correct power handling ability as well as the corrrect resistance.