Honestly if you are very new to this sort of thing I would more so recommend simply creating a work-around for this process, your own registration script can simply add multiple user records (one for each service's user table -- mind you the WordPress login you may want to be careful with considering giving everyone access to your CMS may not be a good thing), then create a user table for your own login purposes that simply reference the appropriate records from each of the other user tables. This way when you login, you identify the user based on one user table by pulling the appropriate records associated with the current login from the other user tables.
The only tricky portion of this process would be in the actual login process, this is where you would need to identify the appropriate function that processes a login request on behalf of those services, or you can attempt to simply do more of an HTTP request on the login scripts already supplied for each of the services. Either method would take a little bit of research, but its far less than making an attempt to alter the code altogether to work differently than it was originally intended. This isn't a process I would recommend for someone who doesn't have the experience. Your bound to end up with a million-and-one headaches throughout the process, and most likely will cause breaks in the original scripts your using since making one change will most likely affect other areas of the system, and so on and so forth throughout the entire services script.
Hopefully I'm not being too direct with this one, but it would be my suggestion. Did notice that someone is offering assistance to help you alter the scripts in an effort to integrate, but I still don't recommend obtaining his/her help (simply because what will happen when that nice individual is no longer there and you are faced with the need to make changes to your own scripts, which inadvertently require further changes inside of the pre-mades? it wont be pretty).
Good luck, if you want my further advice on such a research process I'm more than willing to offer any further advice. Just let me know.