Custom Function for Mapping a Field Across Two Modules

Custom Function for Mapping a Field Across Two Modules

I would like to know if there is a function that will help me map a date field (Demo Date) from one module (Accounts) to another (Deals). I know I can include this field as part of the native Lead conversion process, but this date field is not typically filled in until after the Lead is converted as it is associated with a deeper part of the sales process. The field is not a Lookup field, which is where I've seen other posts on a similar topic. Neither of the Modules are Custom Modules, either. Is this possible? If so, could you please provide the script?