1. Read about Autofill here: http://wiki.mementodatabase.com/index.php/Library_Edit_screen#AUTOFILL_tab
2. Read about using the Http object here: http://wiki.mementodatabase.com/index.php/Memento_JavaScript_Library#Processing_an_HTTP_request
3. Read examples of the Http object here: http://wiki.mementodatabase.com/index.php/Trigger_Examples#Http_Examples
You may choose Autofill, if it sits you. That would be simplest and probably would not involve any scripting. If scripting _is_ needed, or if you have your own Web site to parse, look up Custom Data Sources in the wiki.
If you have greater needs, the Http object might help, but you'll need to wow choice for that.
If plain Autofill, which works only from certain given Web sites, doesn't suit you, but coding up JavaScript and trying to use an Http object is beyond you, read this: http://wiki.mementodatabase.com/index.php/Data_Sources. Just a little, relatively simple JavaScript can be used to satisfy a need like that. You would be on your own to fetch data from a Web site that was not designed as a data source, and then you can use JavaScript like the first example (where another library is used as the data source) to plug the data into the new entry.
I'll mention that Memento has a Table View, but I think you're wanting to parse tables on Web sites, which is an entirely different thing.
Hopefully, someone with a clue will respond to this.