I added an "import grades" button to the gradebook page. It is a bit crufty and the format isn't documented on there anywhere, but it does work. It only takes changing two lines in a blackboard-style assignment CSV to make it moodle-compatible. I've tried it out with uploads from Web-CAT and things are working OK.
The upload format is a simple CSV file like this:
username,### smith,27 jones,43 james,78
Here, the entries in the first column are the student PIDs (i.e., Moodle login user names), and the second column contains scores (scores get rounded to integer values in Moodle, I think). The ### in the first line should be replaced by the assignment number into which you are uploading the grades. If you hover your mouse over one of the column titles in the gradebook view, you'll see that those are links directly to the assignments, with a url that looks like:
Take the id number at the end of this url and use it in the spreadsheet's first line. Note that the spreadsheet need not contain an entry for every user. User names not found will show errors. The upload simply replaces the specified user name's score for the specified assignment, for the set of users in the uploaded file.
If you want to read more about it, see this thread: http://moodle.org/mod/forum/discuss.php?d=12015#67295.
If you are downloading scores from Web-CAT and wish to upload them in Moodle, make sure that "omit course staff" and "use Blackboard format" are both checked in Web-CAT, then click the download button. Save the resulting CSV on your machine. Open up the CSV file and do the following:
Delete the last line (which gives the total number of points available).
Replace the first line by changing the first column entry to be "username", and changing the second column entry to be the Moodle assignment number, as described above.
Don't worry about floating point values in the Web-CAT data (if it is there). It will be rounded (not truncated) when you upload to Moodle.