You can. But you have to escape such special characters like a newline or an escape (0x1b) character with some special char sequences like:

• *N subsitutes to a newline (0x0a)
• *E subsitutes to a esc char (0x1b)
• ** subsitutes to a *
• *" subsitutes to a "

For example the following command would write a string to a texteditor containing a newline:

'WRITEEDITOR "TEXT Hello Joe,*N I would like to meet you."'

Please note that this applies to all Arexx command arguments and that you have to use proper escape sequences like explained above.