Q: "How do I convert 15-character IDs into 18-character IDs?"

A: Create a formula field with a name like "Opportunity ID (18 char)" that returns text. The formula should be: CASESAFEID(Id)

You can also install Excel Connector and then use the FIXID() function in a cell, like this:

The 15-character ID was cut-and-pasted into cell C1, and the formula in cell D1 translates that ID into an 18-character ID.

If you're more code-savvy, navigate here for the source code to convert IDs in C#, Visual Basic, and Java. Or here to see how to do it using a wacky formula function.