How to make a custom field become a new available placeholder in send-out Email

Hello mates,

I am a newbie and now trying to setup a little online store via WooCommerce, the Checkout Manager plugin really help me a lot, but still there are some issues I try to solve but can’t find right method in the internet, so I creat this topic ask for help, hope can find out a solution, here is the situtaion:

Firstly, I want to know my customers’ gender, so I create a new billing field at the plugin(Name:billing_gender, type:select, label:Gender,options:Mr /Miss /Mrs), let customer could select their gender in the checkout page.

Then I want to use this new added field could be use as a placeholder/variable in the all auto sending out email, for example: {customer_gender}, just like the other pre-built avaliable place holders such as {site_title},{order_number},{customer_last_name}.

Fore example, a new customer is a gentlemen name Peter Parker, So that I could make something like the following in the email body text:

Html code:
Dear {customer_gender) {customer_last_name}, Thank you for your support and purchase, please check your order details and order number #{order_number}, have a nice day!

Text in Email:
Dear Mr Parker, Thank you for your support and purchase, please check your order details and order number #12345, have a nice day!

Any suggestion what I should do to achieve this? (code add into function.php also ok)