One "solution" is to store them on large, inexpensive PC disk drives rather than directly in the IFS. A Windows shared network PC drive can be accessed through the /QNTC file system from your AS/400. A Linux shared directory can be accessed with NFS by mounting the Linux share.
If stored on a PC, it's probably also a good idea to burn them to CDs or DVDs for archival storage.
However, you could simply have a small AS/400 that needs more or larger disk drives. You should post the model, the version/release of the OS and the number and capacity of your disk drives.
Finally, if the IFS is really "full", your system will come to a stop. If it is still running, then something else is the problem.