need help with "Calculation Express"

Hello! MDM Experts -

Need help writing the formula using the expression editor on a calculated field to do some lookups and fill the value of a field as mentioned below.

Example: If the field value "AddressName" on main table matches the field value "Lookup1Name" in lookup table "Lookup1" at the time of data load (via import manager), then fill the field "AddressCustomerNo" on the main table with "Lookup1CustomerNo" from the "Lookup1" table.

If the value is "Null" or the doesn't match, then do nothing (skip).

All the fileds are type "Text". MDM Version 5.5 SP06 (5.5.63.57).

Look forward to hearing on any creative ideas/solutions. Thanks!