Thank you for your message.
Well, the easiest way to import subscribers si to use the built-in subscriber import tool -> link text
There is a section Import subscribers right in the Newsletter tool. You can see that there is a textarea where you can type in the subscribers - this is what you can use. According to valid expressions:
You can make an SQL query for your other database to get results in this format and then simply copy and paste it there.
Let me know if you have further questions.