Does Apple's Numbers support 3rd party plugins / data providers / formulas / functions?
No.
According to this outdated post the answer is no. I also doubt Apple would add this feature.
There is no "plug-in" architecture yet for Numbers. the '09 version does have some basic scripting ability it looks like. But I haven't gotten it to work yet. Waiting for someone that knows more than me to post some examples.
You can make a feature request here
Although not plugins per se, Numbers (and the entire iWork suite) does have a library for AppleScript.