zhaop,
The structure model within FrameMaker is designed to have paragraph-containing elements, each of which contains a single paragraph. In your case, it sounds like you are using the table cell as the paragraph-containing element which is OK, but doesn't work so well when you want multiple paragraphs in a cell. So, I think your best bet would be to add some additional elements as children to the table cell, each of which delimits an individual paragraph. For example, something like:
<cell>
<p>Here's a paragraph</p>
<p>Here's a paragraph</p>
<p>Here's a paragraph</p>
</cell>
You might be able to do it with linefeed and carriage returns somehow but I wouldn't recommend it. Using structural markup would be much more reliable.
Russ