How to append extra content to email and send it bith customer and admin

Hi,

Where does the extra content comes from ? The customer enters it somewhere ? Or you enter it ? Is it static for everyone for all the products or dynamic ?

If it's a static text you want to add to the emails, you could use translation overrides to add your text to the text already in the email:
www.hikashop.com/download/languages.html#modify

If it's more dynamic, then it will probably require editing the email via the menu System>Emails (Business edition only) to add some code to the email based on what you need exactly.

Also, I don't understand what you mean by " is there any way to get admin email in my plugin". You want to send an email from a plugin ? An email already in HikaShop ? Or you mean you want the email address of a super account user ? Or something else ?