how user will add emoji's in the UI and need to store in DB

You can work with unicode characters for the emoji. Store the unicode data in db and to show it in UI do some string manipulation like below

a!localVariables(
  local!emojiData:"\U0001F600",
  a!richTextDisplayField(
    value:char(hex2dec(local!emojiData))
  )
)

If user wants to add emojis by selecting it in UI then you need to look for any open source emoji API and using that you can have a list of emojis for user selection. 

Similar thread from the past community.appian.com/.../saving-emoji-and-special-characters-to-appian-cloud-database

When storing emojis to DB, you have to make sure to set the tables and fields to the UTF8MB4 collation. Then, you can store 4-byte characters. Appian cloud DB is set to 3-byte by default.