There is no way to assign a specific drive letter to CD-ROM drive.
In OS/2 drive letters are assigned as the drivers are encountered in
the CONFIG.SYS. Well sort of, .DMDs are assigned first, then .SYSs.
You can adjust to agree what drive letters are assigned by moving
statements around in your CONFIG.SYS.
The /R parameter someone else mentioned will not help you in this
particular case.