The easiest way is to do this is to use the spreadsheet as your data source. You have to create a new database file, and set it up as a connection to your spreadsheet. There are instructions on how to do this in the Getting Started With Base
manual. (Note that the link is a download link for the pdf). Look under "Accessing Other Data Sources." Or, you can take a look at this thread in the forum
Note that connecting to the spreadsheet will not allow you to edit the data in the Base document, but it will be quicker for your purposes. The forum thread above also mentions a way to import the data into Base so you can edit it within base.
Then you can access this data in Writer for envelopes.