There is a way to assign weights to your customer records in ENVISION. You simply need to select appropriate geocoding settings while importing your customer file.
Under Record Definitions section select Total Population as a Base Variable and select Record Weight under the second drop-down menu. Once you select Record Weight, another drop-down menu will appear. You will enter the field name for the weight.
Please click on the link below to access a detailed QuickStart document on customer file import.