When do you add the extra a when using sa? Ex: Sa or sa a se pou mwen.

You, actually, can have the "a" in front of "sa" when it acts as a demonstrative.
examples:

1. kay sa a → this house
2. Bagay sa a se tèt chaje. → this  thing is a headache.
3. Mwen vle sa a. → I want this. / I want this one.
4. Ou mèt pran sa a. → you may take this one.
5. Sa a pa pou vann. → This is not for sale.
And here's one I heard recently thanks to the educator Rony Joseph
I truly forgot that people still talked like that:
6. Moun sa a yo pa gen kè. → These people don't have hearts.

Instances where you can't have this "a" are, for example, when "sa" means "what"
examples:

7. Sa w vle? (Kisa ou vle?) → What do you want.
8. Sa l genyen? → What's wrong with him?
9. Se sa mwen vle. → That's what I want.
10.  Sa sa a ye la? → What is this?
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words