Importing Members from ExpressionEngine
Tricky is a mild way of putting it.

EE and Craft both hash (not encrypt) their passwords differently. Hashing differs from encryption in a few ways, but the most important one here is that hashing is not reversible, which means that the passwords cannot be recovered. Which, in turn, means that if you import the user information into Craft, the members still would all have to reset their passwords.

If that's okay, then you would be looking at a custom plugin that uses the Users Service.