You seem to be confused between Licences and Ratings. What licence do you actually hold?
If you have an EASA licence with a TMG Class rating or priviledge then all you need is a Class rating Skill Test to add the SEP Class rating, the licence remains the same.
If you have a NPPL (SLMG), that is not a TMG rating or licence and you will need to convert the licence, provided you obtained it before 8th April 2018. There is no point adding anything to it because you cannot convert anything obtained after April 2018.
I suggest you are lookig to convert your NPPL (SLMG) to a LAPL TMG in which case CAP 804 Part P Para 3.3
Conversion of UK National Licences and ratings not included under 3.1 or 3.2
1. Hold an NPPL(A) with SLMG rating;
2.Hold an LAPL medical certifi cate (or a Class 1 or
Class 2 medical certifi cate) issued in
accordance with Part MED (MED.A.030). Any
limitation arising from the holder’s medical
status will appear on the medical certifi cate;
3. RT Licence if required
4. Have knowledge of the sections of Part OPS
and Part-FCL relevant to the LAPL(A) with TMG
privileges – in accordance with A.1(b) of Annex
II to the EASA Aircrew Regulation;
If the applicant has not completed 6 hours pilot
in command of TMGs since being granted the
NPPL(A) with SLMG rating, comply with
FCL.105.A(b) – LAPL(A) Privileges and
Conditions on issue of the Part-FCL LAPL(A) in
order to be entitled to carry passengers, i.e.:
Then add the SEP privileges to the LAPL.
It can be done by a CRI but only at a ATO or DTO.