using PHPMyAdmin or another MySQL GUI Tool, you could export the information from the Power Store users table to the power messenger users table.
if both power store and power mailer use the same databse, you could also use the following query:
INSERT INTO pm1_users (UserEmail,
UserPassword,
UserFirstName,
UserLastName,
UserCity,
UserState,
UserZip,
UserEmailVerified,
UserRegistrationDate,
UserVerificationCode,
UserIP,
UserPhone,
UserFax,
UserCountry,
UserAddress,
UserAddress2,
UserGroupID)
(SELECT UserEmail,
UserPassword,
UserFirstName,
UserLastName,
UserCity,
UserState,
UserZip,
UserEmailVerified,
UserRegistrationDate,
UserVerificationCode,
UserIP,
UserPhone,
UserFax,
UserCountry,
UserAddress,
UserAddress2,
UserGroupID
FROM ps4_users)
if you already have users in the PowerMailer database though, this will not prevent duplicates