You won't be "converting" anything, you'll be getting your ATP. For that you need a CPL/IR - either your FAA or EASA will do the job. You may or may net need TSA clearance and a visa
First you need to stand an ATP CTP course which is similar to an MCC course.
Then you can take your ATP written exam
Then get flight training in a piston twin as required to take the ATP test. You may be able to find a SIM provider that will let you take the test on type, but you won't get the type rating unless you do the full course.