Load the style which you wish to copy into the Composer Memory (usually A) then copy this style to a Custom Memory location - it doesn't matter which one you use. Load the Composer with the style you wish to retain in Memory A, then using the Custom to Composer copy facility, copy the style you wish to add to your orginal, from your chosen Custom Location, into Composer Memory B.
You can use the Composer to Custom / Custom to Composer facility, to make up your own mix of styles, either in Custom or Composer memory. Remember to save your new Composer set-ups to SD Card. You can also edit any of the parameters of the style, whilst it is present in the Composer Memory.

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Willum