First, remember that a CD only holds about 650mb of information where as I assume your backup save tape probably has more than that on it.
In theory, a CD can be used to restore/IPL an "as/400". Recall that when the risc boxes came out in the 1990's, there was a CD drive and as I recall, one could do such from cd.
Your question is probably along the lines that can you do such? that I can't answer.
Today folks are looking at multiple DVD's, etc. to hold the backup.
So, you don't have a DVD-RAM drive?
This is fairly old (2000): CD-ROM Premastering on iSeries, but the APIs still exist, so it should still work. It talks about several different types of CDs you can create, and SAVSYS is one of them. But for details of SAVSYS (specifically), it references another document.