A REXX EXEC can directly call a program which resides in the HFS with the spawn() syscall. Following is an example of a REXX program that can be called from TSO/E.

/* REXX */
RC = SYSCALLS('ON')
If RC<0 | RC>4 Then Exit RC
Address SYSCALL
fstdout = 'fstdout'
fstderr = 'fstderr'
'open' fstdout O_RDWR+O_TRUNC+O_CREAT 700
stdout = RETVAL
'open' fstderr O_RDWR+O_TRUNC+O_CREAT 700
stderr = RETVAL
map.0=-1
map.1=stdout
map.2=stderr
parm.0=1
parm.1='/bin/c89'
'spawn /bin/c89 3 map. parm. __environment.'
spid = RETVAL
serrno = ERRNO
If spid==-1 Then Do
  str ='unable to spawn' parm.1', errno='serrno
  'write' stderr 'str'
  Exit serrno
End
'waitpid (spid) waitpid. 0'
xrc = waitpid.W_EXITSTATUS
If xrc^=0 Then Do
  str =parm.1 'failed, exit status='xrc
  'write' stderr 'str'
End
Exit xrc