Here are a couple of options depending on the context:
Phonetic Approximation:
* ஜோஹன்னா (Jo-han-na): This uses the Tamil letter "ஜ" which can be pronounced as a "j" sound in some contexts. However, it's not a perfect match.
* ஜோனா (Jo-na): This simplifies the name using the "ஜ" sound.
Meaning-based:
* யோவானா (Yo-va-na): This translates to "God is gracious" which is similar to the Hebrew origin of "Joanna" (meaning "God is gracious").
Ultimately, the best way to spell "Joanna" in Tamil depends on how you want to pronounce it and the desired level of accuracy.