I don't know about scripts or utilities to do this, but ...

If you want to move users, there is a script from John Mitchell that will import users from an XML file (see here).  You will need to import user information from your existing portals.  Passwords might be a problem, but you can assign random passwords and require users to change them the next time that they login.

As for moving pages, I'd recommend that you export your existing sites, and then use the template to import the pages into your existing site.  You'll probably have to do some editing.  You'll need to make sure to export content, and remember that the content will only be exported from modules that support exporting.  If there are other things that don't transfer, you can probably figure out how to move data from tables in the old site to the new site ...

Depending on the number of sites you intend to consolidate and the amount of data, the difficulty of doing this can range from "not too hard" to "a real mess"!  <g>