The extention depends on which kind of table types you have set for the db.
Files with '.frm','.myd','.myz' are MyISAM type (which is the default type in ver 3.23)
and '.frm','.isd','.ism' are ISAM type.
So you should be able to use MyISAM tables in UNIX as well as ISAM.
Otherwise you can change the table type with a "ALTER TABLE xyz TYPE=ISAM" query.
I have only converted the other way, so I don“t know what risks involved around it. There is a comment in the online manual about type conversion and symlinked tables.