With Customizer...No.   You will need the SDK for your version of Sage 500 which includes utilities for working with Lookup controls and what they display and retrieve.  

It depends on the scope of your intended change.

Lou might be thinking of modifying the results in a specific form, but the query and some of the properties of a lookup can be modified using the Maintain Lookups and Maintain Lookup Views tasks in System Manager. The data returned to the lookup is based on the view with which it is associated, with metadata properties stored in the data dictionary. The SDK makes it easier to import new schema into the data dictionary but it is not necessary to create or modify a the base view of a lookup or to add it dynamically to a form using Customizer. What is required is an in-depth understanding of some of the code structures, methodologies and tools available in Sage 500 to perform these types of changes, since some modifications have the potential to render the function or feature inoperative. You also need to understand how these types of features are reused and can be be modified throughout the application code base.

For example, the ship method lookup is used in a number of different forms like Set Up Shipping Methods, Enter Sales Orders and Quotes, Enter Purchase Orders and probably at least a dozen others since it is also embedded in many report and BI filter grids. You can limit results or apply certain types of changes to the lookup either from the administrative level (using SM tools), or within form code, but you need to have some expertise in how the application uses these features for any significant modification to the logic.