Hi Saddam 

Thanks for posting in the community! You can access Greek letters by uploading a custom font into your Lucidchart account. Please visit this help center article to learn more about how to upload custom fonts. 

It would help if the official comment stated which font might accomplish the task.

Hi Emilia! We are experts on Lucidchart but unfortunately not on all external fonts please let us know if you have any questions about uploading fonts and we'd be happy to help! 

This site seems helpful for bringing in symbols -- I used it to add arrows to my process box text.

http://xahlee.info/comp/unicode_index.html