Yes – you can open a Lifetime ISA if you already have other types of ISA, as long as you are eligible for the account.
Remember: your personal ISA allowance of £20,000 per tax year is shared across all of the types of ISAs that you have, (across other banks too). The Lifetime ISA allowance of £4,000 per tax year counts towards this overall £20,000 per tax year ISA allowance.
One other thing to note is that you can have both a Help to Buy ISA AND a Lifetime ISA, but when the time comes to buy a home, you can only use the bonus from one of these types of account towards the purchase. For more info on transferring the funds from your Help to Buy ISA into a Lifetime ISA, read this article.