Yes you are for a direct import. Joomla's salting mechanism is pretty decent, so don't bother try breaking it (you can, but it'll take a while to do it for all 250 users).
Instead, what you could do is import all the records into the other database with the exception of the password column (Leave it blank).
Then, write a plugin that on a successful login into Joomla will check the other DB and update the password there...
I'm not familiar with Prestashop, but you should be able to write some code that would check for a blank password when they login, and present a message to login to Joomla first.
Either that, or write a single-sign-on system so that all authentication goes through Joomla (which is what I would do, since then you only have 1 place to manage users, instead of two)...