Genghis,
I'm just about to start doing that myself for someone at the end of the week, in his case it's actually NPPL(M) to SSEA, so I'd be interested to see what you come up with. There are, as you've probably seen, some minimum requirements in the NPPL cross-crediting document.
NPPL(Microlight) or UK PPL (Microlight) to NPPL (SSEA)
The holder of a valid NPPL with Microlight Class Rating or UK PPL (M) licence without restrictions who wishes to obtain a SSEA Class Rating shall:
a. Produce the NPPL or UK PPL (M);
b. Produce logbook evidence of currency on Microlight aircraft;
c Carry out such SSEA conversion training as is judged necessary by the FI(A) or CRI(SPA) conducting the training to achieve the required standard for the applicant to take the NPPL NST and GST in a SSEA. This training must include:
(1) Not less than 1 hour of dual instrument appreciation;
(2) 2 hours stall awareness/spin avoidance training;
(3) Differences training for Microlight pilots whose Microlight flying has been solely on flexwing aircraft;
(4) Not less than the 32 hours required minimum total flight time for the NPPL with SSEA Class Rating, which may be a combination of both Microlight and SSEA flying.
d. Pass the JAR-FCL PPL (A) theoretical examination in Aircraft (General) & Principles of Flight;
e. Hold a valid NPPL Medical Declaration or JAA Class 1 or 2 medical certificate;
f. Pass the NPPL NST and GST in a SSEA.
However, I guess it's "c" specifically "conversion training as is judged necessary.....to achieve the required standard" that's the crux of your question. I was planning to do something similar to what Whopity suggests. Basically, do a GST, see how good (or bad) it is and then carry out further training on what exercises require it.