E.g. we have SP 2010 application along with PeopleSoft application. Now we want to add users from PeopleSoft application to the User profile and merge with existing users with UPS database. What are the ways to do this?
you can write PowerShell script or Timer Job and execute it periodically. In code you can read information from PeopleSoft for example. Then check if such user already has profile, just update it, if user hasn't create new user profile. Look at this blog post for information how to create new user profile programmaticaly.