I want to get 2 more ip from isp 's DHCP server by virtual interface.
I think it’s not possible with RouterOS. I had this issue before and had to manually asign static ip’s. I’m not sure if it’d work, but if you really need dhcp, you might try putting a switch to split your incoming connections to two ports of RB. If you have only one port to spare, then - RB250GS and VLANs.
Is it able to implement by RB450G? I am not able to get the IP under VLAN, even I try to assign the different mac address by bridge.
As I haven’t tried it in practice I’m afraid to mislead you with my theories. I’m not sure but you should be able to assign few vlan interfaces to your wan port of RB, but they must have different vlan id’s, so it leaves everything to rb250gs magic to put everything in the same segment.
After all, tomorrow is monday, so perhaps some rested and fresh heads will suggest something better and easier 
Maybe this:
http://wiki.mikrotik.com/wiki/Manual:Interface/Virtual-ethernet
I haven’t tried this, I just googled it 
It is for virtual machines running inside ROS. Not going to do any good in this situation..
Hope to find the solution too, I don’t want to waste the port of RB but share same bandwidth.
(extra power for the switch and use all the port for this)
This might be a quite good suggestion to a feature request list. I’m sure there are times when iface alias might be useful. I did use them on linux long time ago. Don’t remember what for though..
it is able to apply this feature on the virtual machine , but not able to bring it out to the RB it self.