Yes, there is a way to capture passwords so that users can keep their old passwords.
First, you'll have to make sure that all users have been imported into Infusionsoft and assigned all the necessary tags, etc.
In the designated password field in Infusionsoft, assign a value that contains the following: #$i4w$capture$#
You can (and should) add something in front or at the end of that string, but it must contain that specific sequence of characters.
Once you've done that, you will need to add the following line near the top of your wp-config.php file:
After that, as your users attempt to login, their passwords will be captured and stored in Infusionsoft.