There is no such thing as a 'NPPL(M)' as such. The NPPL(Aeroplanes) can include one or more of the SSEA, Microlight and SLMG Class Ratings.
In effect you wish to credit your current total SEP time towards the NPPL(A) with Microlight Class Rating.
More information can be found both on the NPPL website and in CAP 804, which becomes effective on 17 Sep 2012,