To access the box drawing and special characters on CP-437 from within
the Memo Editor, Press and hold:

HP 95LX		[ALT] and then enter the number
HP 100LX 	[ALT] [MENU] then the number

For example 177 is the code for the Grey Box. 	
Once you have the characters you want to use, you can just highlight
and copy and paste as many as you want.
I like to keep a small text file with the ones that I use often and some
pre drawn boxes.