Hey there

Thank you for posting in the Lucid community!

Our sincere apologies for the delayed response on this thread - I wanted to follow up in case this answer may still be useful to you and to provide a solution for other users who see this thread in the future. 

You can import a UML mark up sequence by typing (or pasting) your markup directly into Lucidchart as explained in the following article: UML Sequence Diagrams made easy

Please let us know if you have any additional questions!