In Microsoft Dynamics NAV 2013 it is supported (and default value) to use text variables without a length.(not fields!)
just my 2 cents:
Because of this it could be possible to use a workaround for a „memo field“ without an add-in.
1. Just create a page (or use an existing one) with a text variable without a length.
2. Use the variable instead of a field directly in the page.
3. Set some properties about the look and feel (like width) and .
voila‘ „a memo control“
All you have to do now is add some code for storing the data in a blob (in the table) and
– reload it when you get the record or
– save it (you can use an action for this) when you want to change it.
Of course no text formatting options are available.