There are some alternative MessageLog.xsl files available which transform the Messenger XML logs into Plus! text logs... With this you can open any Messenger log and save it again as a Plus! text log.
search forums for "MessageLog.xsl"...
PS: but note that the Log Viewer of Plus! Live will also already read the xml logs from Messenger though. So you don't really need to convert anything (unless you want them to be in the same format as the Plus! xhtml logs)