I think what you mean is that you want to add a microlight class rating to your NPPL. The UK PPL(Microlight) ceased at the time the NPPL was introduced. Information can be found here: http://www.laa.uk.com/
NPPL (SEP), UK PPL (A) or JAR-FCL PPL (A) to NPPL (Microlight)
The holder of a NPPL (SEP), UK PPL (A) or JAR-FCL PPL(A) with SEP class rating who wishes to obtain a NPPL (Microlight) shall:
a. Produce the NPPL (SEP), UK PPL (A) or JAR-FCL PPL (A);
b. Pass the Microlight Aeroplanes Type (Part 2) oral examination conducted by a Microlight flight examiner. The examination shall include pilot maintenance requirements and conditions of the Permit to Fly;
c. Hold a valid NPPL Medical Declaration or JAR-FCL Class 1 or 2 medical certificate;
d. Pass the NPPL (Microlight) GFT.
So you will need to go to a Microlight school to achieve this: http://www.bmaa.org/clubs.asp