Can I use international transfers to qualify for a retirement extension in Thailand instead of having 800k฿ in the bank?

would not work because you need to be ON the visa/extension you're using monthly income for while you're transferring the funds in.

You can show proof of funds in your country to get the initial 90 day Non-O and then you'd have to have the 800K in a thai bank account in your name only for 2 months before you apply for the yearly extension.

Now once you get the initial 90 day Non-O you can start moving in to the country a minimum of 65K baht a month transferred from abroad into a thai bank account in your name only and use the for the FOLLOWING year's extension (after you met the requirements for the first year by banked money method)