You can fit resistors but really why bother? What is the problem with the oem bulbs?
Anyway, to calculate the correct resistance required measure the resistance across the old bulbs using a multimeter. Measure the resistance across the new led (record the lowest reading as it will vary depending on the direction of current). Then subtract the led value from the filament bulb value. This will give you the additional resistance required in the circuit to convince the CANBUS all is good. Buy the resistors from Maplin or such like (they are well cheap). It will be worth purchasing a few of differing resistance as the calculation above gives a ball park figure based on the resistance with a small current from the multimeter. The current in the real circuit will be higher so the filament bulb will have slightly higher resistance. This shouldn't be significant but the test above will give you a ball park figure so you can purchase a range of resistors with appropriate resistance. If you want to know precisely the resistance then you will need a working circuit with the oem bulbs and you should measure current by putting your multimeter in series and the calculate resistance by dividing voltage by current. Once you know the correct resistor required then wire each one in series with the appropriate bulb.
Alternatively buy some bulbs rhs work.