You will need to get the information into Dynamics-CRM for it to be included in your email.
You need to create the username and password attributes in your customer (account or contact?) entity, this would be done by customising that entity.
Then you'd need to import it from the excel spreadsheet. CRM can do imports from Excel but getting it to update the correct customer records would be difficult, I think you'd need the GUIDs for each CRM customer in the excel spreasheet so that CRM could make the connection. The alternative is to write code that accesses the Dynamics CRM webservice, search for each customer and set the custom fields.
Once you have the data in CRM it should be fairly easy to create a quick campaign and add the fields in to the body of the email, just as you would for the customer name or other details.
The alternative would be to export from Dynamics-CRM into Excel, join up the data and do a much simpler mailmerge from there, but this would stop the emails and their responses being tracked, which is kinda the whole point of CRM.