PL/SQL Developer, by Allround Automations, does this automatically. It turns this:

some_line_of_code
some_line_of_code
/* some comment about code */
some_line_of_code
some_line_of_code

into this:

/*some_line_of_code
some_line_of_code
\* some comment about code *\
some_line_of_code
some_line_of_code*/