The expression se payer la tête de means se moquer de quelqu’un / to make fun of somebody.

The en plus part adds something to the sentence to render that the person is probably not just making fun of somebody, as in “adding insult to injury”.

The sentence can be understood as:

Is he making fun of me as well / on top of that?

You can find se payer la tête on Wiktionary.

payer ma tête – > to make fun on me

en plus – > You were correct with the meaning of in addition to. Here, in addition to… what the subject had done or said previously.

But this en plus can also implicitely mean that the subject does obviously not qualify for making fun on me.

These two meanings could also be expressed with c’est le comble as in that’s the last straw displaying some… irritation.

English could say : on top of that.

Il est en train de se payer ma tête, en plus => On top of that, he is making fun on me.

En train de

Indeed, it conveys the idea that “il” is currently doing the action later mentioned in the sentence. Something commonly carried by the continuous tenses of English.

Se payer ma tête

First, the expression will be “se payer la tête de quelqu’un”. The pronominal use of the verb changes its meaning slightly. The meaning of “se payer qqch” in this case is “to treat oneself with smthg”:

• Je me suis payé une journée de repos → I treated myself with a day off

How do we treat ourselves with someone else’s head, though? Well, it’s a bit of a going back and forth in the human body structure. First, the head is the physical support of the face. Then, the face itself is a valid representation of a person as a whole: the whole body would perhaps be more complete, but the face is enough in a lot of cases, even on legal photo ID’s, where other body features can be put as words or numbers. And so the word tête in the expression represents the whole person.

And so we treat ourselves with someone else. There could be several ways to indulge in this kind of pleasure, but the sole meaning retained in French for this expression has to see with actively enjoying the situation someone gets him- or herself into:

• by relentlessly teasing the person

or

• by actively/noisily/showily making fun of their acts or words.

En plus ?

It is used as an exclamation that expresses surprise, sometimes a pleasant one, sometimes not so much. There is also, as mentioned in the other answers, the idea of something added (plus) to an already existing circumstance.

For example if someone not only prepared a fastuous meal, but also some ridiculously decadent dessert as well, in English one could say: “…and you even made that dessert on top of it all!?”. A French equivalent could be: “…et en plus, tu as préparé ce dessert !?” or “…et, tu as préparé ce dessert en plus !?”

In this case, we can expect something happened to me (whoever the me of the example is), and on top of perhaps injuring myself, or being booted out of somewhere, or burning my car by mistake, or looking like a fool talking about my soon-to-be-ex-girlfriend while not realizing she moved in the room, or whatever else unpleasant happened to me, I have to cope with people teasing me.

Of course, it doesn’t have to be that negative. My pants could have come down while I was juggling in the park, and I could be laughing at myself just the way my friends do, but feign indignation to increase everyone’s hilarity.

The expression se payer la tête de quelqu’un is stronger than se moquer de quelqu’un.

The latter is close to openly “making fun of someone” while the former is a softer variant of se foutre de la gueule de quelqu’un where most notion of “fun” has disappeared, replaced by lies, cheat, betrayal and then often implies the victim is unaware of it, at least on the beginning.

An (American) English translation might be to mess with someone or even to fuck with someone.

I would then try to translate:

Il est en train de se payer ma tête, en plus ?

with:

Plus, isn’t he messing with me behind my back?