First, create the file to be used. It should be a single block on the drive. This will create a file (swapfile) of size 100 MB as a large block.

dd if=/dev/zero of=/swapfile bs=1024 count=100000

Next, add the newly created file to the swap pool.

mkswap /swapfile

Finally, turn on the new swapfile.

swapon /swapfile

Now you have 100 more Mbytes of swap memory.