I'm pretty sure there is no automated migration, and ACM and SIM are different enough that you would probably want to perform a migration project. First, configure SIM to match what is required in your current ACM (settings, users, products, libraries, etc.). I would note that SIM has extra capabilities over ACM so you may want to take advantage of those during this configuration phase. Second, download all content from ACM. Third, upload all content to SIM. That is obviously a simplification, and there possible shortcuts along the way, if you are not familiar with these tools then you should probably engage a specialist to help.
