Multiple SMS, Marketing campaigns through iOS?
Although its not exactly what you were asking for, this App does exactly what you want. It seems a little expensive, though. 

Answering your question, there are a couple of ways you could import the contacts to the iPhone. You could, for instance, do it by:


Saving the file as .cvs
In Outlook Express, choosing File / Import / Address Book, and opening the .cvs file.
Connecting your iPhone and in iTunes, tick the box next to Sync Contancts With: and choosing Outlook Express.


Afterwards you could use an App such as Group SMS, or Group Text! or even create your own group to send the SMSs. To do so, just:


Open the SMS App.
Tap the New Message icon, in the top right corner.
Add all the recipients you want to send the SMS to by tapping the + sign.
Compose and send the message.


A Conversation will appear in your Sent Messages list. Therefore, if you want to send a message to the same group, you'll just have to compose the message there and send. This will be a bit painful if the recipient's list is a thousand names long.