extend the modules/Leads/clients/base/layouts/convert-main.php layout via the Extension Framework

mapNewFields.php

<?php

//File: /custom/Extension/modules/Leads/Ext/clients/base/layouts/convert-main/mapNewFields.php

//One Gotcha with this, this file will be required
//when metadata is being rebuilt and so you must avoid defining the following
//variable names here: $module and $type

//Doing so will override those variables in the MetaDatFiles parser and cause
//un-expected results.

//The $customMapping array contains the extra set of fields, for each module, that 
//should be mapped with values from the source Lead record
//
//For example, in the mapping below, we have extra mappings for each of the Modules
//involved in the Lead conversion process. The Keys of the $customMapping must be the module
//Name that corresponds to one of the defined Modules in the Layout definition. Each module entry
//contains the mappings for that module. The key (left-hand side) corresponds to the module field and
//the value (right-hand side) is the Lead field, whose value should be mapped to the module field.
$customMapping = [
    //Contacts module field as key and Lead field as value
    "Contacts" => [
        "department" => "new_cstm_field_c",
    ],
    "Accounts" => [
        //Accounts module field as key and Lead field as value
        "new_acct_field_1_c" => "new_cstm_field_c",
        "description" => "website",
    ],
    "Opportunities" => [
        //Opportunities module field as key and Lead field as value
        "description" => "new_cstm_field_c",
    ],
];

foreach ($viewdefs['Leads']['base']['layout']['convert-main']['modules'] as $moduleIndex => $moduleEntry) {

    //Look into custom mapping array for a Key matching the module name being iterated over
    //in the LayoutDef
    if (!empty($customMapping[$moduleEntry["module"]])) {
        foreach ($customMapping[$moduleEntry["module"]] as $destFieldName => $sourceLeadFieldName) {
            $viewdefs['Leads']['base']['layout']['convert-main']['modules'][$moduleIndex]['fieldMapping'][$destFieldName] =
               $sourceLeadFieldName;  
        }
    }            
}