Native fields in Lead mapping for conversion

For Leads conversion process Salesforce provides out-of-box functionality for mapping Lead fields to Account/Contact/Opportunity.

Unfortunately there is a limitations - you can't map native Leads fields to target object's native field. For instance you can't map value from LeadSource ("native" Lead field) to AccountSource ("native" Account field).

Here is one of the possible ways to workaround the limitation. Solution could be modified based on particular business requirements. Please do not hesitate to contact us if you want to leverage this solutions in your Saleforce instance.

STEP 1: Create custom Formula(Text) field for Lead object
Name: LeadSourceCustom
Formula:  TEXT(LeadSource)

STEP 2: Create custom Text(100) field for Account object
Name: AccountSourceCustom

STEP 3: Amend Lead fields mapping to write value from LeadSourceCustom to AccountSourceCustom.

STEP 4: Create simple Trigger on Account object

/*
    Version        : 1.0
    Company        : Websolo inc. 
    Date           : 07/2016
    Description    : 
    Update History : 
*/
trigger AccountSourceTrigger on Account (before update,before insert) {
    for(Account a: Trigger.new){
        if(a.AccountSourceCustom__c != null && a.AccountSourceCustom__c != '' && (a.AccountSource == null || a.AccountSource == '')){
            a.AccountSource = a.AccountSourceCustom__c;
        } 
    }
}

As a result this trigger will update native AccountSource field with value from "native" LeadSource field.

EXTRA NOTE:
We can't use WorkFlow as a workaround because AccountSource field is picklist and when we create workflow action to update this field we can write only specific value from the picklist.