You CAN add a SSEA, Microlight, or SLMG class rating to a UK national PPL and use that with a NPPL Medical Declaration as if it were a NPPL.
Here's the form for a UK PPL holder (see third tick box down on the form) to add a SSEA class rating:
http://nationalprivatepilotslicence....ion%20Form.pdf
providing they comply with the requirements of NPPL cross-crediting:
http://nationalprivatepilotslicence....20REV%2011.pdf
However, you can no longer have a SSEA, Microlight, or SLMG class rating issued onto a JAR-FCL or EASA licence.
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