As your FI is also legally valid on a Microlight, then in theory you could do a FI Microlight test and apply for a separate FI Microlight qualification
If your FI rating is current, you don't even need to do a flight test to obtain a Microlight FI rating, you can just apply to The Pod for issue on the basis of your existing qualification.
In the case above the FI rating is not current, so a flight test is required whichever route you choose.
ifitaint...