While Extensis is good at exporting all metadata, importing data into Lightroom is liable to be the problem. Lightroom has no built-in tool to import text data, though there is a plug-in called LRTransporter, and I have ListView.
Even with these plug-ins, you require a fair amount of skill and patience, and will have to deal with text file formats (encoding) and glitches caused by non-English characters. Also the metadata itself may have no corresponding place in Lightroom. For example, if you have a multiple value custom field in Extensis, what does that become in Lightroom? Lightroom doesn’t provide custom fields, so you’d be reliant on a plug-in, but even plug-in custom fields can’t have multiple value. So you’d probably want to make your Extensis multiple values into Lightroom keywords.
If you have JPEGs and TIFs, making Portfolio embed the metadata in the files may be the easiest approach for most folk. For custom fields, you might use Extensis’s excellent XMP mapping to point those fields to IPTC fields.
If you have raw files, PSDs, or other files where Extensis won’t embed metadata…. One route may be to make temporary JPEG and TIF copies of those files using Extensis – with the same metadata as the corresponding proprietary files (can’t remember if this can be done without scripting). Next you would import the real and the temporary files into Lightroom. You can then use my Syncomatic plug-in which copies metadata between files whose names match but whose extensions differ. So it would copy metadata from “1234_temp.tif” to “1234.psd” (it works on thousands of images at a time).