I want to transfer money in from an old ISA I hold with another provider
You can transfer a previous year's ISA without it counting towards your ISA allowance for the current tax year.
If you transfer a previous year's ISA into a Lifetime ISA, it will count towards your Lifetime ISA allowance for the current tax year. This will be considered a qualifying payment and will be eligible to receive the government bonus.
You will still be able to pay in up to £20,000 to any combination of other ISAs during the tax year.