Yes, there is/was a version of mencoder compiled for n810.
Here is the original post:, but the download links no longer work:
http://talk.maemo.org/showpost.php?p...2&postcount=10
May be somebody still keeps it (I had it, but unfortunately I deleted it)
Anyway, it's better to use a pc or you could die of old age waiting for it to convert a video.