You'd have to enable ip forwarding and setup some firewall bit..
echo 1 >/proc/sys/net/ipv4/ip_forward
/sbin/iptables -t nat -A POSTROUTING -j SNAT --protocol tcp --to-source BBB -s AAA --destination-port 21
where BBB is the ip address of hostb on hosta's network.
and AAA is the network of hostc's network.
...then when you type in
ftp://hosta on the hostc computer it'll go to hosta.
If you want the ftp to go to hosta when you type
ftp://hostb
you may want to try some other program like "redir".