Can I increase my payment to Humble Bundle after the bundle has ended and receive BTA?

Nope.

No.

Yes. (up to 30 days after the bundle ends)
Can I add more to my payment after the bundle ends?
http://support.humblebundle.com/customer/portal/articles/243214-can-i-add-more-to-my-payment-after-the-bundle-ends-

As others have said, nope. A shame, I wanted to grab the Weekly and another bundle today, but other stuff took priority.

You might be able to try trading, but be careful of scammers.

Hm, most people say nope and a few say yes :) This up to 30 days option to increase your payments might be the key, but this would only work for our own purchases and not gift ones too. Thanks for the replies so far, we'll see if someone would appear here who tried it and got the BTA games after the end of a bundle. I guess that's the only way to be 100% sure anyway :)

Yes you can.
Here's a quote from their site:

If you have already purchased a bundle, you can add to your original amount during the bundle sale period (and up to 30 days after the bundle ends). Just go to your personal download page and there will be a link that says "Like what we're doing? Increase your order amount".

It states that you can increase the payment up to 30 days AFTER the bundle has ended.

yes i always buy at the beginning when the bta price is low for 1$ and then when the bonus come out i decide what to do