When you create a title, it receives the same succession law as your primary title. So if you adopt Primogeniture then create the Kingdom of Sicily, the kingdom receives Primogeniture.
Note that this is true if you create the title, but it is not true if you usurp the title. (Usurped titles will normally revert to the default succession law - eg. ag-cog gav for feudal christians.) If a title already exists, it might be worthwhile to force it to be destroyed so you can recreate it, instead of direct usurpation.