There is really no easy way to answer that, if you have a fixed price Direct Debit. Something like rent then you can just setup a recurring bill in the money out section .
But if you have varying amounts then you’ll need to manually enter it, or simply don’t enter it and when you’re doing your bank import just create a bill from the import amount.
So, when you import something from a bank account to Bullet say DD €345, Bullet will look to see if that DD €345 is in it’s system. If it’s not will will ask you would you like to create a bill/invoice for that.
That’s probably the easiest thing to do.
Hope that helps