You should ask your host to install AbiWord as well. Then you can call the exec() function and use this line:
AbiWord --to=html
You'd have to run AbiWord --help to get the full syntax, but my guess is it's:
AbiWord --to=html /home/username/public_html/file.doc
Hope that helps. And in case you're wondering where I'm getting my information from, it's right on wvWare's project page under the wv Utilities heading.