Visiting Trianon Palaces & Domaine de Marie Antionette

Entirely possible, separate ticket is available.

Is the entry point different from the main entry at Versailles?

Yes, it is different, further north. A picture being worth a thousand words, I'll kindly redirect you to the Château de Versailles website (chateauversailles.fr) that has all the info you need.

You can enter the main gardens through the regular entry at the chateau. That is a separate entry from that of the palace itself. From there you would go through the formal gardens to the Grand Canal, then over to the Trianons. Or you can go through town and enter the gardens via the Queen's Gate. Depending on where you are coming from, it might be faster. On a fountain day, you would bypass the paid zone of the gardens via the Queen's Gate (but you'd have to leave that way too).

On fountain days when you have to pay to enter, you need a ticket to enter the gardens regardless of which gate. On other days anyone can enter at any gate; it is free. The wider estate is also free every day although you need a ticket for the Trianon and Hameau (although we entered the Hameau without one -- ). These spots are only open in the afternoon and you can buy a ticket at the Trianon.