Automatic excerpt to protect content...

I'm planning to use the membership plugin for a near project and would like to know if it's possible to use an automatic excerpt to protect content from non members.

The shortcodes and the more tag do their job correctly but you have to manually set them inside the posts/pages, if you have a lot of articles to upload than this can be a tedious task...the automatic excerpt should be adjustable in word count or number of lines (whatever is possible to do , I guess word count but you might be able to make it work in another way...).

Thx.