There are a couple of options:
1. Make your note, then draw a rectangle around the desired text. Problem is that the box doesn't move with the text.
2. Make your note, then create a "Box" style balloon ("Tight Fit" size) containing enough spaces to match the length of the text you want to enclose. Line the balloon up over the portion of the note you want to enclose, select balloon and note, RMB->Group. Now the box and the text move as a unit. If you change the note text you can un-group, move the box, and re-group.
Neither is great, but that's about all I can come up with.
-handleman, CSWP (The new, easy test)