yes - you can use an external modem, just hook the ethernet cable up from the modem to the port on your MOBO .
If you are talking about another nic other than the one that is integrated then you will have to disable the on board one through bios, unless you are using your system as a dhcp server then you can use one as input and the other as output .. but if you have them both enabled and just using one, what could happen is that the system will get confused and not know which to use, even if you select one and disable it in the network settings, and you will get packet loses ..
If you use USB you will lose speed, it is recomended that you don't use sub ..
That is what you wanted to know right ?? the way you have your question is a little confusing lol ..