It should be no problem at all. Put the plugin into 3rdParty/TinyMCE4/plugins folder.
Add tinymcespellchecker to plugin: in the wysiswyg_custom.php
plugins: "contextmenu,table,fullscreen,paste,media,lists,advlist,charmap,textcolor,
link,anchor,hr,paste,image,code,imagetools visualblocks,searchreplace,tinymcespellchecke",
Jeff Shields