Their website makes no mention of Linux support.
I looked at the Wine applications database
and there is no mention of Castup, though that could be because no one has reported on it.
A native Linux version does not appear to be available. You could try installing a Windows virtual machine (VirtualBox is in the Ubuntu repos--you would need to supply your own Windows CD) and installing Castup in it, but I grant that that's cumbersome workaround.