Yes, of course you can do this. Use mtools
for the FAT and e2tools
or genext2fs
for the ext2 filesystems.
$ dd if=/dev/zero of=/tmp/disk1.img count=1440 bs=1k
1440+0 records in
1440+0 records out
1474560 bytes (1.5 MB) copied, 0.00569719 s, 259 MB/s
$ /sbin/mkfs.msdos /tmp/disk1.img
mkfs.msdos 3.0.12 (29 Oct 2011)
$ mdir -i /tmp/disk1.img
Volume in drive : has no label
Volume Serial Number is 9913-BFF6
Directory for ::/
No files
1 457 664 bytes free
$ mcopy -i /tmp/disk1.img /etc/issue.net ::/
$ mdir -i /tmp/disk1.img
Volume in drive : has no label
Volume Serial Number is 9913-BFF6
Directory for ::/
issue net 28 2012-06-26 10:49
1 file 28 bytes
1 457 152 bytes free
And of course, I have no root rights.