Learn Spanish (Latin America): Ella a veces practica yoga en el parque. - She sometimes practices yoga in the park.

Yes! Adverbs of frequency like a veces (sometimes) are quite flexible. You can put it at the very beginning (A veces ella practica...), between the subject and verb like in this sentence (Ella a veces practica...), or at the end (Ella practica yoga en el parque a veces).

Is a veces always two separate words?

Yes, it is always written as two words: a and veces. A common mistake is to write it as one word, but that is incorrect in Spanish. Literally, it translates to "at times".

In English we often say "does yoga". Can I use the verb hacer here instead of practicar?

Absolutely. Both practica yoga (practices yoga) and hace yoga (does yoga) are very natural and common ways to say this in Latin American Spanish.

Do I have to include Ella in this sentence?

No, you can drop it and just say A veces practica yoga en el parque. However, because practica can mean "he practices", "she practices", or "you (formal) practice", keeping Ella makes it instantly clear exactly who you are talking about.