combining ajax.net with existing javascript

Well, my instincts tell me that you probably can, but it's probably undocumented. If I were going to try and do it, I'd look at the HTTP stream from Fiddler to see what's being passed back and forth. I'd also say that if it were me, I'd do it w/o an UpdatePanel. There's plenty of other options if you look at PageMethods, WebMethods, the Sys.Net js namespace, and one nice little hookus from Scott Guthrie on his blog in case you want to push complex controls down easily: http://weblogs.asp.net/scottgu/archive/2006/10/22/Tip_2F00_Trick_3A00_-Cool-UI-Templating-Technique-to-use-with-ASP.NET-AJAX-for-non_2D00_UpdatePanel-scenarios.aspx

Thanks, that link is exactly what I have to do. It's like ajaxPro except the content is generated on the server.
glad I could be of help.