You need to first export the Customer Master Data into a file format of your preference, be it Excel, Access database table or even a csv file.
Assuming that your customer numbers are always numeric (no alpha characters... ever) and you that have less than a million customers, you should be alright with the .xslx format.
Next in the Sales model build an external lookup (in the Table window). Step through the wizard, supplying the file that you want to link to, the common fields in both data sets that will be used to find the matching/related data, and the fields from the other file that you want to bring into the Sales model.
There's a [URL="http://********************/sources/video-monarchs-external-lookups"]video demo[/URL] available if you want to see this in action.