Before proceeding make sure you have backed up.  I tried this using a separate directory and installation of XMWin distinct from either of my working copies of WSWin or XMWin.  The only actual working file which I modified was xmwflt.ini in the Windows directory.

The XMWin spell checker files are:

 xmwlang.dll
 icdllwin.dll
 ihdllwin.dll
 redllwin.dll
 pddllwin.dll

 henis321.dat
 iens9522.dat
 rektr300.dat
 reutr300.dat
 

The WSWin spell checker files are:

 wswlang.dll
 icdllwin.dll
 ihdllwin.dll
 etdllwin.dll

 ienc.dat
 hecdp.dat
 rentf.dat
 

To install the WSWin spell checker in XMWin rename XMWin's icdllwin.dll and xmwlang.dll to *.org.  Copy the WSWin spell checker files into the XMWin directory.  Rename wswlang.dll in the XMWin directory to xmwlang.dll.

Now the WSWin spell checker will run in XMWin.  If you want to switch back, restore the icdllwin.dll and xmwlang.dll to the originals.