How do I add leading zeros to a variable in Sharepoint 2010 site workflow?

I'm using Sharepoint 2013, but this particular site workflow is a 2010 version. I'm trying to accomplish this in Sharepoint Designer.

This workflow creates a new document, and assigns a given number to it. To determine the number I'm retrieving a numeric value from a particular list. After getting this value I need to add leading zeros to it before concatenating it with another string. For instance, the value I get back might be 4, and I want it to have a total of 3 characters, so in this case 004. A value of 12 should be formatted 012, and so on.

An alternate approach I tried was using calculated fields rather than putting this together within the workflow. Unfortunately this isn't a comprehensive solution for me as I need the formatted value available as a Word Quick Part, and as far as I know calculated fields cannot be used for this.