I am currently using phplists as my mailinglist program and would like to tie the XOOPS db with the phplists db...

Phplist allows me to add additional attributes to the user db any time I want and therefore as I proceed in time sending my mails out, I can adjust the db as needed.
This is the best mailing list I have ever seem and would hate to loose it.

I don't think the guests to my website want to register twice: once to use the site and then again to add their email to the newsletters and mailing list. Therefore, I would like to find a way that these guests sign up only once for both items when they so desire.

How do I do this?

Personally, I would keep the two separated.

There would be way to much coding involved to integrate the two user DB's into one finely tuned section. You would have to adjust your tables, and find a way to import users from system to the next. Thats beyond my scope of knowledge.

Now, the reason I Would keep them separated is because lets say for instnace a user does join your site. If they like and want more, you can provide a link to your mailing list, where they can sign up.

This gives the end user the added advantage of security and it protects you from getting blamed for spam or unwanted mail. Since they have to actually go to a different link and manually add the email address of their choosing, it is a more secure and safe environment for all involved.

Essentially, users who really want to be on the mailing list have to do the extra steps to achieve what they want. You can explain to them that this is just a security measure for all your members and insure that only peple who really want to be on the mailing list are getting on the list.

Thats just my two cents.